Poston, 33, squandered his advantage thanks to four bogeys in the first 13 holes, with England’s Tommy Fleetwood briefly taking the lead on the 15th.
However, Fleetwood bogeyed the 17th and finished in a tie for fourth with American Sam Burns on 10 under par, while American Wyndham Clark, who shot a 67, was third.
American world number one Scottie Scheffler, aiming to equal Tiger Woods’ record of winning three consecutive titles, finished 12th alongside England’s Justin Rose and Northern Ireland’s Rory McIlroy.
Play was suspended on Saturday because of thunderstorms, with the third round concluding on Sunday morning.
Poston, who was tied at nine under with Gerard on the sixth green when play was halted, moved to 12 under, with Gerard four shots back.
Burns was third on seven under, with Fleetwood and Clark – on six under – completing the top five after three rounds.