PHILADELPHIA — No matter how the flow of the game is going, there is one stat where Derrick White almost always leads the Boston Celtics .  Plus/Minus.  But a weird thing was happening as Game 3 wore on in Philadelphia . White’s plus/minus kept dropping deeper and deeper into the negative, even as the Celtics kept rebuilding six-to-10-point leads. The normally reliable White wasn’t just going cold, he was starting to struggle with his matchup with Tyrese Maxey.  When he missed a layup with 4:34, a play that was sandwiched by an Andre Drummond corner three and a Kelly Oubre Jr. layup, the unthinkable presented itself.  Maybe it was time to sit him down.  “Anybody that ever doubts D-White, they don't really care about winning,” Joe Mazzulla said after the game.  That lesson was hammered home immediately. The Celtics came out of the timeout called after Oubre Jr. 's layup and got a great look in the left corner by Nikola Vucevic.…