The Bruins trailed 3-1 entering the third period, but Frederic cut it to 3-2 at 5:18, knocking home a loose puck bouncing in the slot, and then Marchand tied it 3-3 at 14:38, picking up Justin Brazeau's clearing pass off the wall and skating in to beat Kolosov between his pads.
Zacha scored in overtime for the second time in three games, finishing David Pastrnak's backdoor feed after keeping the puck in the offensive zone with a heavy forecheck.
"It was a long battle in the corner, and I think that's something when you win those battles, you can be one guy," Zacha said. "That's who usually scores. When 'Pasta' had the puck on his stick, I was trying to just get open and it was a great play. But the battle was very important."
It's the second straight game the Flyers blew a third-period lead. They led 5-4 against the Florida Panthers on Thursday but allowed three goals in the final 5:17 of a 7-5 loss.
"We've got to learn from our mistakes," Flyers forward Sean Couturier said. "This is two games where we take stupid penalties, give the other team a chance to come back, and we did it again today."
Michkov gave the Flyers a 1-0 lead at 7:24 of the first period with a power-play goal. Emil Andrae's point shot was blocked down into the slot, where Couturier flicked it out to Michkov inside the right face-off circle.
The Flyers went ahead 2-0 at 18:24 when Michkov scored off Sanheim’s cross-ice pass.
The goal gave Michkov three straight multipoint games (three goals, four assists).
"[Michkov] has got a knack for scoring," Sanheim said. "He wants to score goals, he enjoys scoring goals, and [he] puts himself in good positions to score goals. So that's just the player that he is. It's a pretty easy play by me to find him when he's sitting that wide open."