Paetsch and Hecht scored in a 4-minute span early in the second period to put Buffalo up 2-0. Paetsch scored during a delayed penalty, firing a slap shot from the high slot. Hecht's shot from the left faceoff circle got through MacDonald's legs.
Tlusty's score, midway through the second period, pulled Toronto to 2-1. Miller made the initial save off Tlusty's backhand, but the puck trickled through.
"I didn't mean to do that, but I count every goal," Tlusty said. "It doesn't matter how it went in."
McCormick put Buffalo up 3-1 on a swat of a loose puck late in the second period, but Mitchell, 94 seconds into the third, made it 3-2 on a tap-in from the slot.
Toronto outshot Buffalo 10-2 in the third period, but couldn't get the equalizer past Miller.
"We were putting it to them pretty good in the third period," MacDonald said.
Buffalo concludes its preseason schedule with road games in Montreal on Saturday and Toronto on Sunday.
The Maple Leafs have three games left. They will visit Detroit on Friday before hosting Detroit and Buffalo on Saturday and Sunday.
Copyright by STATS LLC and The Associated Press
Arena: First Niagara Center Location: Buffalo, New York
Attendance: 11,008 (58.9% full)
No scoring this period
Nathan Paetsch (with extra attacker) Assist: Jason Pominville, Philip Gogulla