UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the
Securities Exchange Act of 1934

Date of report (Date of earliest event reported): March 24, 2026

Cartesian Growth Corporation III
(Exact name of registrant as specified in its charter)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company  x

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act.  ¨

As previously disclosed, on December 17, 2025, Cartesian Growth Corporation III, a Cayman Islands exempted company (“Cartesian III”), Fenway MS, Inc., a Delaware corporation (“Merger Sub”), and Factorial Inc., a Delaware corporation (“Factorial”), entered into a Business Combination Agreement (as it may be amended, supplemented or otherwise modified from time to time, the “Business Combination Agreement” and the transactions contemplated thereby, the “Business Combination) and furnished a presentation to be used by Cartesian III and Factorial in presentations for potential investors and other persons regarding the Business Combination. Cartesian III is furnishing an updated form of investor presentation that is attached as Exhibit 99.1 and incorporated by reference herein. Cartesian III and Factorial may use such presentation in meetings with potential investors and other persons regarding the Business Combination.

The information in this Item 7.01, including Exhibit 99.1, is furnished and shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to liabilities under that section, and shall not be deemed to be incorporated by reference into the filings of Cartesian III under the Securities Act or the Exchange Act, regardless of any general incorporation language in such filings. This Current Report on Form 8-K will not be deemed an admission as to the materiality of any information in this Item 7.01, including Exhibit 99.1.

The disclosure set forth above in the first paragraph of Item 7.01 of this Current Report on Form 8-K is incorporated by reference herein.

Forward-Looking Statements

This Current Report on Form 8-K includes forward-looking statements. Forward-looking statements generally are accompanied by words such as “believe,” “may,” “will,” “estimate,” “continue,” “anticipate,” “intend,” “expect,” “should,” “would,” “plan,” “predict,” “potential,” “seem,” “seek,” “future,” “outlook” and similar expressions that predict or indicate future events or trends or that are not statements of historical matters. These forward-looking statements may include, but are not limited to, statements regarding future events or the future financial or operating performance of Factorial or Cartesian III. For example, projections of Factorial’s future financial performance, manufacturing capabilities and operations, Factorial’s business plans, and other projections concerning key performance metrics or milestones are forward-looking statements. Such forward-looking statements are subject to risks, uncertainties, and other factors which could cause actual results to differ materially from those expressed or implied by such forward-looking statements. These forward-looking statements should not be relied upon as representing Cartesian III’s and Factorial’s assessments as of any date subsequent to the date of this Current Report on Form 8-K. Accordingly, undue reliance should not be placed upon the forward-looking statements. Neither Cartesian III, Factorial nor any of their respective affiliates undertake any obligation to update these forward-looking statements, except as required by law.

Additional Information about the Business Combination and Where to Find It

This communication relates to the proposed business combination between Factorial and Cartesian III (“Business Combination”) pursuant to that certain Business Combination Agreement, dated as of December 17, 2025, by and among Cartesian III, Fenway MS, Inc., a Delaware corporation, and Factorial. The proposed Business Combination will be submitted to shareholders of Cartesian III for their consideration. Cartesian III and Factorial intend to file a registration statement on Form S-4 with the Securities and Exchange Commission (“SEC”), which will include a definitive proxy statement to be distributed to Cartesian III’s shareholders in connection with Cartesian III’s solicitations of proxies from its shareholders with respect to the proposed business combination and other matters to be described in the Form S-4, as well as the prospectus relating to the offer of the securities to be issued to the stockholders of Factorial in connection with the completion of the proposed Business Combination. After that registration statement has been filed and declared effective, Cartesian III will mail a definitive proxy statement/prospectus and other relevant documents relating to the proposed Business Combination and other matters to be described in the registration statement to Factorial stockholders and Cartesian III shareholders as of a record date to be established for voting on the proposed Business Combination. Before making any voting or investment decision, Cartesian III shareholders, Factorial stockholders, and other interested persons are urged to read these documents and any amendments thereto, as well as any other relevant documents filed with the SEC by Cartesian III in connection with the proposed Business Combination and other matters to be described in those documents when they become available, because they will contain important information about Cartesian III, Factorial and the proposed Business Combination. Shareholders will also be able to obtain free copies of the preliminary proxy statement/prospectus, the definitive proxy statement/prospectus and other documents filed by Cartesian III with the SEC, once available, without charge, at the SEC’s website located at www.sec.gov, or by directing a written request to Cartesian Growth Corporation III, 505 Fifth Avenue, 15th Floor, New York, New York 10017.

Participants in the Solicitation

Cartesian III, Factorial, and their respective directors and executive officers may be deemed to be participants in the solicitations of proxies from Cartesian III’s shareholders with respect to the proposed Business Combination and the other matters set forth in the proxy statement/prospectus. Information regarding Cartesian III’s directors and executive officers, and a description of their interests in Cartesian III is contained in Cartesian III’s final prospectus for its initial public offering filed with the SEC on May 5, 2025, which is available free of charge at the SEC’s website located at www.sec.gov, or by directing a request to Cartesian Growth Corporation III, 505 Fifth Avenue, 15th Floor, New York, New York 10017. Additional information regarding the interests of such participants in the proxy solicitation and a description of their direct and indirect interests, will be contained in the proxy statement/prospectus relating to the proposed Business Combination when it becomes available. Shareholders, potential investors and other interested persons should read the proxy statement/prospectus carefully when it becomes available before making any voting or investment decisions. You may obtain free copies of these documents from the sources described above.

This communication is not a substitute for the registration statement filed by Cartesian III or for any other document that Cartesian III and Factorial may file with the SEC in connection with the proposed Business Combination. INVESTORS AND SECURITY HOLDERS ARE URGED TO READ THE DOCUMENTS FILED WITH THE SEC CAREFULLY AND IN THEIR ENTIRETY WHEN THEY BECOME AVAILABLE BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION. Investors and security holders may obtain free copies of other documents filed with the SEC by Cartesian III, without charge, at the SEC’s website located at www.sec.gov.

No Offer or Solicitation

This communication shall not constitute an offer to sell, or the solicitation of an offer to buy, or a recommendation to purchase, any securities, in any jurisdiction, or the solicitation of any vote, consent or approval in any jurisdiction in connection with the proposed Business Combination or any related transactions, nor shall there be any sale of securities in any states or jurisdictions in which such offer, solicitation or sale would be unlawful. This communication is not, and under no circumstances is to be construed as, a prospectus, an advertisement or a public offering of the securities described herein in the United States or any other jurisdiction. No offer of securities shall be made except by means of a prospectus meeting the requirements of Section 10 of the Securities Act, or exemptions therefrom. INVESTMENT IN ANY SECURITIES DESCRIBED HEREIN HAS NOT BEEN APPROVED BY THE SEC OR ANY OTHER REGULATORY AUTHORITY NOR HAS ANY AUTHORITY PASSED UPON OR ENDORSED THE MERITS OF THE OFFERING OR THE ACCURACY OR ADEQUACY OF THE INFORMATION CONTAINED HEREIN. ANY REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.

(d) Exhibits

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

Date: March 24, 2026