Reading jumped out to a 1-0 lead in the first inning when center fielder Tyson Gillies walked and third baseman Cody Asche and left fielder Darin Ruf followed with singles.
Designated hitter Jake Fox, batting cleanup, also singled, but Reading failed to score the rest that inning. Reading managed just two singles the rest of the game against six Trenton pitchers.
Right-hander Trevor May started for Reading and allowed one run -- a fifth-inning homer by Addison Maruszak -- on three hits while walking two and striking out six in 5 1/3 innings.
Right-hander Kyle Simon followed May and tossed 2 2/3 scoreless innings with three strikeouts. Losing pitcher Colby Shreve issued a one-out walk in the ninth that was followed by a single, wild pitch and an intentional walk to load the bases. Shreve struck out first baseman Kevin Mahoney, but Segedin followed with his game-winning triple.
Right-hander Ethan Martin (5-0) starts for Reading on Friday at 7:05 p.m. ET.
The winner of the series advances to the best-of-five championship series. Bowie leads Akron, 2-0, in the other semifinal series.