UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 3)*

3SBio Inc.

(Name of Issuer)

Ordinary Shares, par value $0.0001 per share

American Depositary Shares, each representing seven Ordinary Shares, par value $0.0001 per share

(Title of Class of Securities)

88575Y105

(CUSIP Number)

OrbiMed Advisors LLC

OrbiMed Capital LLC

Samuel D. Isaly

601 Lexington Avenue, 54th Floor

New York, NY 10022

Telephone: (212) 739-6400

Copies to:

Warren S. de Wied, Esq.

Wilson Sonsini Goodrich & Rosati

1301 Avenue of the Americas

New York, New York 10019

Telephone: 212 999-5800

(Name, Address and Telephone Number of Person

Authorized to Receive Notices and Communications)

April 15, 2013

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box:  x

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule 240.13d-7 for other parties to whom copies are to be sent.

The information required on 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).

1

Introduction

This amendment no. 3 (“Amendment No. 3”) relates to the Ordinary Shares, par value $0.0001 per share, of 3SBio Inc. This Amendment No. 3 is filed jointly by OrbiMed Advisors LLC, a limited liability company organized under the laws of Delaware, OrbiMed Capital LLC, a limited liability company organized under the laws of Delaware, and Samuel D. Isaly, an individual (collectively, the “Reporting Persons”) to amend and supplement the Items set forth below of the Schedule 13D previously filed by the Reporting Persons with the Securities and Exchange Commission (the “SEC”) on April 8, 2013 (the “Schedule 13D”), as amended by Amendment No. 1, filed with the SEC on April 11, 2013, and Amendment No. 2, filed with the SEC on April 12, 2013. Except as provided herein, this Amendment No. 3 does not modify any of the information previously reported on the Schedule 13D. Capitalized terms used but not defined in this Amendment No. 3 have meanings provided in the Schedule 13D.

Amendment No. 3 is filed to amend Item 4 and Item 7 of the Schedule 13D as set forth below.

Item 4. Purpose of Transaction

Item 4 of the Schedule 13D is hereby amended by adding the following:

On April 15, 2013, the Reporting Persons issued a press release, a copy of which is attached as Exhibit B.

Item 7. Materials to be Filed as Exhibits.

Item 7 of the Schedule 13D is hereby amended and restated in its entirety as follows:

2

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.

Dated: April 15, 2013

EXHIBIT INDEX

Exhibit A

JOINT FILING AGREEMENT

The undersigned hereby agree that the Statement on this third amendment, dated April 15, 2013 (the “Amendment No. 3”), to the Schedule 13D, dated April 8, 2013, with respect to the American Depositary Shares, each representing seven Ordinary Shares, par value $0.0001 per share, of 3SBio Inc. is filed on behalf of each of us pursuant to and in accordance with the provisions of Rule 13d-1(k) under the Securities Exchange Act of 1934, as amended, and that this Agreement shall be included as an Exhibit to this Amendment No. 3. Each of the undersigned agrees to be responsible for the timely filing of the Amendment No. 3, and for the completeness and accuracy of the information concerning itself contained therein. This Agreement may be executed in any number of counterparts, all of which taken together shall constitute one and the same instrument.

IN WITNESS WHEREOF, the undersigned have executed this Agreement as of the 15th day of April, 2013.

Exhibit B

OrbiMed Advisors Announces Institutional Shareholder Services Recommendation that 3SBio Shareholders Vote AGAINST Management Buyout Proposal

NEW YORK—(BUSINESS WIRE)—OrbiMed Advisors LLC (“OrbiMed”) today announced that Institutional Shareholder Services (“ISS”), a leading provider of proxy research and corporate governance services to institutional shareholders, has recommended that shareholders of 3SBio (NASDAQ: SSRX) vote AGAINST the company’s $15.40 per ADS going-private acquisition by a consortium of buyers that includes the company’s Chairman and CEO and other senior executives.

In its report, ISS casts doubt on management’s reliance, in part, on its financial advisor’s earnings-based valuations because they were based on earnings estimates provided by management that are considerably below Wall Street consensus estimates. ISS said, “Shareholders should note that in its analysis, Jefferies used the internal estimates provided by company management, which, in this case, may be inherently motivated to downplay the earnings projections.”

The report further questions management’s commitment to a transaction with a single private equity firm. ISS said, “Further, the exclusivity agreement between Dr. Lou and CITIC PE may have limited the company’s ability to attract a better offer, and there is no compelling evidence that 3SBio cannot continue to thrive as a stand-alone company.”

In response to this latest proxy voting recommendation, OrbiMed said, “Two leading global proxy advisory firms, ISS and Glass, Lewis & Co., have now had a chance to review the 3SBio going-private transaction and, not surprisingly, when considering all the facts, they have reached the same conclusion as we have, that a flawed buyout process and inherent conflicts have resulted in a management-sponsored going-private transaction at an inadequate price that does not serve the interest of all shareholders.” OrbiMed, a 9.4% holder of 3SBio shares, reiterated its intention to vote against the proposals at the company’s April 25 Extraordinary General Meeting.

About OrbiMed

OrbiMed Advisors LLC is a leading investment firm dedicated exclusively to the healthcare sector, with approximately $7 billion in assets under management. OrbiMed invests globally across the spectrum of healthcare companies, from venture capital start-ups to large multinational companies. OrbiMed’s team of over 60 professionals manages a series of private equity funds, hedge funds, royalty funds and other investment vehicles.

Contact:

OrbiMed Advisors LLC

Samuel D. Isaly, Managing Partner

212-739-6400

Georgeson Inc.

David S. Drake

212-440-9861