This
web page by Alexander Selby says that this question was posed by Mike Caro in 1997, and the closest preflop matchup is ATs vs 33, .007% advantage to the pockets if one of the threes is the same suit as the A-T.
See the same page for a discussion of how to find the closest nontrivial three-way matchup, for a few different definitions of "nontrivial."