A day after tying the course record with a 61 to build a four-stroke advantage at Harbour Town, Merritt had four birdies and a double bogey to reach 14-under 199.
Kuchar, Brendon Todd and Kevin Kisner were tied for second.
Todd made the big move with a 63, the day's lowest round. Kisner shot 67, and Kuchar 68.
Jim Furyk led a group another stroke behind after a 68, with Spieth five shots back, also following a 68. Spieth believes he can shoot even lower Sunday to chase a second straight jacket, this one tartan.
