How I see this playing out:
Game 4: Boston wins and goes up 3-1
Game 5: Golden State wins an all-time great game back in SF. Boston up 3-2
Game 6: Closing out + Experience of GS prove a bad combo: Golden State wins in Boston. Tied 3-3
Game 7: There is no way GS loses a Game 7 at home. Golden State wins title 4-3 after being down 3-1.
I don't think the Celtics have lost back to back games these playoffs, but as a full roster haven't lost consecutive games since January 19th/21st. They lost a back to back in March without Brown, Tatum, and 2 other starters sitting the second game.