The Leafs down the Bruins 42 and now trail the series 21
Auston Matthews scored the go-ahead goal in the second period as the Maple Leafs defeated Boston 4-2 to cut the Bruins' lead in their first-round series to 2-1.
Patrick Marleau, with two, and James van Riemsdyk also scored for Toronto, while Mitch Marner and Morgan Rielly added two assists each.
Frederik Andersen made 40 saves to get the win.
Adam McQuaid and Zdeno Chara replied for Boston. Sean Kuraly had two assists for the Bruins, who got 26 stops from Tuukka Rask.
The Leafs will look to even the Eastern Conference quarterfinal in Thursday's Game 4 before the series shifts back to Boston for Game 5 on Saturday.