"He's unbelievable," Raptors guard Kyle Lowry said of Siakam. "He's the most improved basketball player in the NBA this year and he's only going to get better."
Game four in the best-of-seven series is Sunday in Orlando.
Siakam was 13 for 20 from the floor and the Raptors held Orlando to 36 percent shooting.
"It's just taking what a defense gives us and going with it," Siakam said.
Lowry nailed a three pointer with just under eight minutes to go in the fourth to give Toronto a 17-point lead, their biggest lead of the game.
The Raptors scored the first 10 points of the game and led by 11 before finishing with a 26-21 lead at the end of the first quarter.
The first half ended with Ross nailing a 40-foot three-pointer to cut Toronto's lead to 48-45.
The Raptors took a seven-point lead early in the third quarter, but Vucevic converted back to back three-pointers with 6:37 to go for Orlando's first lead of the game, 59-57.
Leonard hit a jump shot to give Toronto a nine-point lead late in the fourth.
The Magic responded and Ross made a layup to cut the margin to 94-90. Siakam's jumper gave Toronto a six-point lead with 93 seconds left.
Ross cut the lead to three with a three-pointer with 42 seconds remaining. Leonard made two free throws with 13 seconds left to ice the victory.
Kyrie Irving helped the Celtics close it out by scoring 11 of their final 16 points.
Boston gets a chance to close out the Pacers in Indiana on Sunday.
Jaylen Brown had 23 points to lead the Celtics, who held the Pacers under 100 points for the third straight game.
Irving finished with 19 points and 10 assists while Jayson Tatum added 18 points, Al Horford had 16 and Marcus Morris 11 in the win.
Tyreke Evans led seven Pacers in double figures with 19 points off the bench. Bojan Bogdanovic had 15 points, Myles Turner had 13, Domantas Sabonis 12, Thaddeus Young and Matthews finished with 11 apiece, and Darren Collison 10 for Indiana. Young was the game's leading rebounder with nine.
Also, Russell Westbrook scored 33 points as the Oklahoma City Thunder beat the visiting Portland Trail Blazers 120-108 to take game three of a Western Conference series.
The Trail Blazers lead the series 2-1 heading to game four on Sunday.