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UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 13G Under the Securities Exchange Act of 1934 (Amendment No. 3)* Comcast Corporation (Name of Issuer) Class A Common Stock (Title of Class of Securities) 20030N101 (CUSIP Number) December 29, 2017 (Date of Event Which Requires Filing of this Statement) Check the appropriate box to designate the rule pursuant to which this Schedule is filed: [X] Rule 13d-1(b) [ ] Rule 13d-1(c) [ ] Rule 13d-1(d) *The remainder of this cover page shall be filled out for a reporting person's initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page. The information required in the remainder of this cover page shall not be deemed to be "filed" for the purpose of Section 18 of the Securities Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes). CUSIP: 20030N101 Page 1 of 4 <Page> 1 NAMES OF REPORTING PERSONS I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY) Capital World Investors ** 2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS) (a) (b) 3 SEC USE ONLY 4 CITIZENSHIP OR PLACE OF ORGANIZATION Delaware 5 SOLE VOTING POWER 208,401,216 6 SHARED VOTING POWER NUMBER OF SHARES NONE BENEFICIALL Y OWNED BY 7 SOLE DISPOSITIVE POWER EACH REPORTING 208,401,216 PERSON WITH: 8 SHARED DISPOSITIVE POWER NONE 9 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 208,401,216 Beneficial ownership disclaimed pursuant to Rule 13d-4 10 CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS) 11 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 4.4% 12 TYPE OF REPORTING PERSON (SEE INSTRUCTIONS) IA ** A division of Capital Research and Management Company (CRMC) CUSIP: 20030N101 Page 2 of 4 <Page> SECURITIES AND EXCHANGE COMMISSION Washington, DC 20549 Schedule 13G Under the Securities Exchange Act of 1934 Amendment No. 3 Item 1(a) Name of Issuer: Comcast Corporation Item 1(b) Address of Issuer's Principal Executive Offices: One Comcast Center Philadelphia, PA 19103-2838 Item 2(a) Name of Person(s) Filing: Capital World Investors Item 2(b) Address of Principal Business Office or, if none, Residence: 333 South Hope Street Los Angeles, CA 90071 Item 2(c) Citizenship: N/A Item 2(d) Title of Class of Securities: Class A Common Stock Item 2(e) CUSIP Number: 20030N101 Item 3 If this statement is filed pursuant to sections 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a: (e) [X] An investment adviser in accordance with section 240.13d-1(b)(1)(ii)(E). Item 4 Ownership Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1. (a) Amount beneficially owned: (b) Percent of class: (c) Number of shares as to which the person has: (i) Sole power to vote or to direct the vote: (ii) Shared power to vote or to direct the vote: (iii) Sole power to dispose or to direct the disposition of: (iv) Shared power to dispose or to direct the disposition of: See page 2 Capital World Investors divisions of CRMC and Capital International Limited collectively provide investment management services under the name Capital World Investors. Capital World Investors is deemed to be the beneficial owner of 208,401,216 shares or 4.4% of the 4,664,327,455 shares believed to be outstanding. CUSIP: 20030N101 Page 3 of 4 <Page> Item 5 Ownership of Five Percent or Less of a Class. If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following: [X] Item 6 Ownership of More than Five Percent on Behalf of Another Person: N/A Item 7 Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on By the Parent Holding Company or Control Person: N/A Item 8 Identification and Classification of Members of the Group: N/A Item 9 Notice of Dissolution of Group: N/A Item 10 Certification By signing below, I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect. Signature After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct. Date: February 8, 2018 Signature: /s/ Michael J. Triessl Name/Title: Michael J. Triessl - Senior Vice President and Senior Counsel, Fund Business Management Group Capital Research and Management Company CUSIP: 20030N101 Page 4 of 4