With 2 games left for both of the Bucks and 76ers, and 1 game left for the Celtics, there are 15 different combinations of possible finishes between the 3 teams.
There is only 1 of 15 scenarios in which the Bucks drop to the 4 seed - lose both, C's win, Sixers win both.
The Bucks win the 2 seed in any scenario in which they win both of their final 2 games. There are 3 out of 5 possible chances for being the 2 seed if they split their remaining games, and 1 of 5 scenarios for being the 2 if they lose both.