It's possible that Phelps could become just the fifth subject to ever grace the covers of Sports Illustrated, Newsweek and Time in the same week. The other four:
1) Secretariat - week of June 11, 1973
2) Mary Lou Retton - week of August 10, 1984
3) O.J. Simpson - week of June 16, 1994
4) 1999 U.S.A. women's soccer team - week of July 19 1999
Note: This is a popular trivia question, but various websites have incorrect answers, at least as far as I can tell. In multiple places Joe Namath, Reggie Jackson and the 1980 U.S. Hockey team were listed as answers, however some quick research on the websites of SI and Time show that those four cover subjects never lined up. (For instance, the Miracle on Ice team was never on Time's cover.) It's also possible that I've missed a cover subject that did make it on all three, so if you know of any other additions to this list, please leave a note in the comments section.