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The third season of the American television medical drama Grey's Anatomy, commenced airing on the American Broadcasting Company (ABC) on September 21, 2006, and concluded on May 17, 2007. The season was produced by Touchstone Television, in association with Shondaland Production Company and The Mark Gordon Company, the showrunner being Shonda Rhimes. Actors Ellen Pompeo, Sandra Oh, Katherine Heigl, Justin Chambers and T.R. Knight reprised their roles as surgical interns Meredith Grey, Cristina Yang, Izzie Stevens, Alex Karev and George O'Malley, respectively, continuing their expansive storylines as focal points throughout the season. Previous main cast members Chandra Wilson, James Pickens, Jr., Kate Walsh, Isaiah Washington and Patrick Dempsey also returned, while previous guest stars Sara Ramírez and Eric Dane were promoted to series regulars, following the extension of their contracts.

The season followed the continuation of the surgical residency of five young interns, as they experience the demands of the competitive field of medicine, which becomes defining in their personal evolution. Although set in fictional Seattle Grace Hospital, located in Seattle, Washington, filming primarily occurred in Los Angeles, California. Whereas the first season mainly focused on the impact the surgical field has on the main characters, and the second one provided a detailed perspective on the physicians' private lives, the third season deals with the tough challenges brought by the last phase of the surgeons' internship, combining the professional motif emphasized in the first season, with the complex personal background used in the second. Through the season, several new storylines are developed, including the arrival of Dane's character, Dr. Mark Sloan, conceived and introduced as an antagonizing presence.

In a departure from the previous season, the third season aired in a new competitive time slot of 9:00 pm on Thursdays, competing against the heavily promoted and highly rated dramatic television series CSI: Crime Scene Investigation, which simultaneously aired on the CBS Network. Season three of Grey's Anatomy contained 25 episodes, in addition to two clip shows that were produced to recap the previous events of the show, before the introduction of major new arcs. "Complications of the Heart" aired on the same night as the season premiere, recapping the last episodes of the second season with insights into future episodes in the third, while "Every Moment Counts" aired before the twentieth episode. The season also aired a two-part episode arc, which primarily served as a backdoor pilot for a proposed spin-off, Private Practice, focusing on the departure of Walsh's character, Dr. Addison Montgomery.

The series ended its third season with an average of 19.22 million viewers per episode and a 6.8/35 Nielsen rating/share in the 18–49 demographic, ranking eighth in the television season, outperformed by CSI. Television critics expressed a mainly negative outlook on the development of the series throughout the season, with the reviews ranging from mixed to unfavorable, as exaggeration and lack of realism have been highlighted as the main issues in the declining quality of the storylines. Despite the negative critical response, the performance of the cast members and the production technique of the crew received outstanding recognition through numerous awards and nominations. Earning major category nominations at the 59th Primetime Emmy Awards and the 65th Golden Globe Awards, the season achieved the series' highest number of recipients, with Heigl being the most awarded cast member. The series was ranked sixth in USA Today's "best of television" list, following the conclusion of the season.