It's too soon to count any of those three remaining games as victories. The Sixers played us tough for stretches last year without Brand; with him injured, perhaps they'll play with more intensity. The Knicks could potentially get hot from the outside; they have a lot of streaky shooters with range. The Bulls have talent, as well.
I expect the Celts to go 3-0; they should be favored by a fairly large margin in each game. However, sometimes great teams drop games to mediocre or worse squads (such as that great Bulls squad losing a game to the expansion Raptors, who finished with 21 wins.)