"We're back in the game now," Leino said. "This was a big confidence-builder for us."
With 5:02 gone in overtime, Chicago goalie Antti Niemi smothered a shot by Simon Gagne that was about to cross the goal line. It was reviewed, and ruled no goal. (Jeff Carter put in the rebound, but it was after the whistle had blown.)
The win sliced the Blackhawks' Finals lead to two games to one. Game 4 will be held Friday at the Wachovia Center.
It was the Flyers' first Finals victory since 1987, ending a seven-game losing streak in championship series. It also snapped the Hawks' seven-game road winning streak, which equaled an NHL playoff record.
The Flyers are 2-4 in overtime games in the Finals. Their only other win was in 1974, when Bobby Clarke's goal - one of the most famous in franchise history - gave them a Game 2 victory in Boston en route to their first Cup.
The Flyers, rebounding from a pair of one-goal losses in Chicago, are 7-18 in series in which they trail, two games to one.
But that's a lot better than the series record (1-6) when they fall into a 3-0 hole.
Climbing out once in this postseason was difficult enough. Doing that against the Hawks would have been virtually impossible.
Chicago took a 3-2 lead when Patrick Kane beat Michael Leighton on a breakaway with 17 minutes, 10 seconds left in the third period. But Leino answered 20 seconds later, depositing a rebound after Giroux's shot bounced off a Hawks defenseman and took a fortuitous carom to the wide-open winger.
With the goal, Leino tied a franchise record for playoff points by a rookie, equaling the mark set by Brian Propp (15 points) in 1980.
Just like in Game 2, the Flyers dominated the third period. They outshot the Hawks by the identical third-period total on Wednesday - 15-4 - as they did on Monday.
Scott Hartnell, not exactly known for his playmaking, made a spectacular pass to set up the only goal of the first period - a power-play score by Danny Briere.
Braydon Coburn's long shot went in and out of the glove of Niemi, and Hartnell pounced on the loose puck to the right of the goal and, while falling down, made a backhanded pass to Briere on the left side of the net.
Briere knocked it in for his 11th playoff goal, tops on the Flyers, with 5:02 left in the first period, igniting the Pennsylvania record hockey crowd of 20,291.
"The best part was when Scott Hartnell found the puck in the slot and just threw it back door. That's part of the chemistry you develop when you play a lot with the same guys," Briere said. "You know where they are going to be, and he knew I was going to be back door."
After Mike Richards barely missed connecting on a pair of power-play chances early in the second period, Chicago tied the game on a goal by defenseman Duncan Keith.
Keith's right circle shot deflected off Carter's stick; the puck sailed over Leighton's left shoulder after just 2:49 of the second period.
The Flyers took the lead on a goal that didn't count until almost two minutes after it was scored.
Chris Pronger's drive, tipped by Hartnell in front, went through Niemi's legs and off his pads before trickling toward the goal line. Defenseman Niklas Hjalmarsson cleared it. The goal light did not go on until five seconds later, and play continued for nearly two minutes without a stoppage.
When there was finally a break in the action, the play was reviewed, and it showed the puck crossing the goal line by a few inches. The goal was counted, and the time that had elapsed was put back on the clock.
There was 8:23 left in the second period when play was stopped, and the goal was actually scored with 10:05 remaining.
That gave the Flyers eight goals in the Finals - four on the power play. Chicago was still looking for its first power-play goal in the series.
Pronger had "a pretty stellar shot. I was able to get a stick on it and just luckily it crossed the line," Hartnell said.
But the Hawks have outplayed the Flyers in even-strength situations, and they scored the equalizer with 2:08 left in the second period.
John Madden won a face-off from Richards and sent the puck back to defenseman Brent Sopel, who, firing from inside the right point, sent a shot past an apparently screened Leighton. Lukas Krajicek, Richards, and Gagne were all lined up in front of Leighton.
With the goal, Chicago had outscored the Flyers, 10-4, in even-strength situations during the Finals.
Through the first two periods, the Hawks had a 21-16 edge in shots, and the Flyers had not played with enough desperation.
They had 20 minutes to change that - or fall into a nearly impossible-to-overcome series deficit of three games to none.
Contact staff writer Sam Carchidi at 215-854-5181 or firstname.lastname@example.org. Follow on Twitter at www.twitter.com/BroadStBull.