Kawhi Leonard, Byrun Mullens or Ray Allen...if they get other things besides 3's that would help too.
Leonard. Because of playing time, he'll only be, at most, 50 less than Allen at 3s, while contributing a TON more rebounds, steals, and overall points.
Ray Allen, most 3's in history of game
If you JUST need 3s, Allen. I'd go with Leonard of the three of them, but Allen could possibly lead the league in 3s.