Goaltender Jean-Sebastien Aubin stopped 30 of 32 shots and picked up his seventh straight win for the Pirates, all by one goal.
Portland now leads the best-of-7 Atlantic Division finals, 3-2. The home team has won all five games in the series to this point.
After a scoreless opening period, the Bruins grabbed a 1-0 lead on a power play goal by Petteri Nokelainen at 8:45 of the second period.
The Pirates tied it at 11:29 on Joe Callahan’s first goal of the playoffs, with Platt picking up the primary assist.
Ryan scored his team-leading fifth goal of the playoffs just 1:33 into the final stanza to give Portland a 2-1 edge, and Platt made it 3-1 with a power play marker at 6:46.
Martins Karsums closed the B’s to within a goal at 11:46, but they were unable to get the equalizer.
Aubin, who did not play in the Pirates’ two losses to open this series, is 7-1 in the playoffs and hasn’t suffered a defeat since Game 1 of the Atlantic Division semifinals at Hartford on Apr. 16.
The AHL regular-season champion Bruins will try to avoid elimination when they host Game 6 of the series on Friday at the Dunkin’ Donuts Center. If necessary, Game 7 is slated for Saturday in Providence.
TORONTO 8, SYRACUSE 2
Division-champion Toronto’s offense came to life as the Marlies cruised to an 8-2 road victory over Syracuse in Game 3 of the North Division finals at the Onondaga County War Memorial.
The Crunch still lead the best-of-7 series, 2-1, and the visiting team has won each game thus far.
John Mitchell was the only member of the Marlies to record more than one goal, as he scored twice and added an assist for a three-point night.
Toronto went 3-for-9 on the power play and scored a total of seven goals over the final two periods of play. The club had combined for only two markers in dropping Games 1 and 2 of the series at home.
Marlies goaltender Scott Clemmensen stopped 27 of 29 shots in the winning effort.