Pelicans were looking for revenge on the road after a 5-0 drubbing at home in game two, and they looked to have every chance when they took the lead early in the second period, when Roni Karvinen worked well in tandem with Aidan Dudas before burying his shot.
But the sides were level again with seven minutes of the period to play, when Joseph Berger found the net after being set up by Vilmos Gallo, and from then on it was all KooKoo.
A roughing penalty against Casimir Jurgens five minutes into period three was punished when Veeti Miettinen played the puck across for Ville Meskanen to put the Kouvola side ahead on the powerplay, and in the final 10 minutes, the same two combined to extend the lead.
Pelicans pulled the goaltender in the final minutes, but the desperation attempt failed when Robert Rooba stroked the puck into the empty net with over three minutes left to complete the 4-1 win. The sides will travel to Lahti for game four on Saturday 4th.
Tappara were shocked on the road at 12th seeds Hameenlinna last time out, but they recovered well by easing to a 3-1 home victory in game three.
It took three-and-a-half minutes for the hosts to break the deadlock, when Olli Juolevi did well to maintain an attack by keeping the puck from crossing the blue line, before darting down the left and shooting from a tight angle to score.
Tempers flared between both sets of players midway through the first period, though it wasn't until midway through the second that the Tampere side doubled their lead, as Kristian Tanus waited and waited for the goalie to move before lifting the puck high into the net.
Tanus then turned provider for Daniel Brickley to add Tappara's third five minutes into the final period, which was more than enough to complete the win, despite a neat deke and finish from Vili Toivonen that got HPK on the board in the final six minutes. It's back to Hameenlinna for game four on Saturday.