Henrik Lundqvist stopped 31 shots to give the Rangers a 2-1 lead in the best-of-seven series. Game 4 is Friday in Philadelphia.
Lundqvist was backed by a defense that delivered with a flurry of blocked shots to stymie the Flyers. The Rangers took an early 2-0 lead and that was enough of a cushion for a team that led the Eastern Conference with 25 road victories.
Mark Streit scored for the Flyers.
Ray Emery struggled in net in his third straight start of the postseason for Steve Mason, and was lifted late in the third period.
Emery started for perhaps the last time this series. Mason hadn't played since he suffered an upper-body injury in a collision on April 12. He got in this game with 7:15 left and the Flyers down 4-1. He'll likely be in net for Game 4.
Coach Craig Berube said Mason was healthy but went with Emery because of his strong first two games. Emery had 31 saves in a Game 2 win.
"Ray just came off a big win. It's not a difficult decision," Berube said.
After a rough outing, it seemed liked the wrong one - right from the start.
Rick Nash's shot wide of the net deflected off the bottom of Emery's glove, and Stepan shoveled the puck in for a 1-0 lead only 3:54 into the game.
St. Louis made it 2-0 midway through the period with a sharp deflection off Girardi's shot from the blue line. The Rangers had two goals on eight shots.
Emery settled down and the Flyers finally generated some offensive pressure, even as leading scorer Claude Giroux failed to get going.
Giroux, third in the NHL with 86 points, had not shots on goal in the first two games and didn't get one in the first period. He did rush the net late in the first, creating space for Streit to score over Lundqvist's stick on a 4-on-4 chance.
Giroux finally attempted a shot about 6½ minutes into the second period and the Flyers down 3-1. The captain clearly needs to start scoring like one of the best in the game for the Flyers to win the series.
Girardi fired a clean slapper high from the point over Emery's left shoulder for that 3-1 lead early in the second.
Down two goals, Jakub Voracek took out some pent-up frustration on Carl Hagelin with a flurry of punches that sent both players to the penalty box and sent the crowd into a frenzy.
Carcillo went after Giroux in the scrum, giving the Flyers the added bonus of a power play. But the Rangers blocked two shots on the power play.
The Rangers closed off the shooting lanes with 20 blocked shots through the first two periods. Lundqvist took care of the rest.
Carcillo sealed the win with a goal soon after he left the penalty box in the third.
Notes: St. Louis had one goal in 19 regular-season games for the Rangers. He has two in the first three playoff games. ... Flyers defensemen have three goals in three games. ... NHL Commissioner Gary Bettman attended. He said the league was close to announcing the teams in next season's Winter Classic and the league would not stage six outdoor games. He also said Islanders owner Charles Wang has received "a number of expressions of interest" in buying the Brooklyn-bound team.