Garcia was at 13-under 129, and it held up as the lead when no one could catch him on a cloudy afternoon with some light rain. Stenson had a 63, and Castro shot 65.
Tiger Woods, in the 1-2-3 grouping with Mickelson and Adam Scott, made a 35-foot birdie putt on the 18th hole for a 67 and was six shots back.
Mickelson, however, managed to steal the show with a 71. Lefty went on the wildest ride at the TPC Boston, completely losing his swing during one stretch.
He closed with back-to-back birdies.
Yani Tseng moved into position to end a long victory drought, shooting a 9-under 63 to take a three-stroke lead into the final round of the Safeway Classic at Portland, Ore.
Tseng had 11 birdies and two bogeys to reach 18 under at Columbia Edgewater.
Rocco Mediate played the back nine in 7-under 29 for the second straight day to increase his lead to two strokes in the Champions Tour's inaugural Shaw Charity Classic at Calgary, Alberta.
The 50-year-old Mediate, the Allianz Championship winner in February in his first Champions Tour start, followed his opening 8-under 63 with a 64 to reach 15 under at tree-lined Canyon Meadows. He had seven birdies on the back nine after making an eagle and two bogeys on the front side.
Former BYU golfer Bobby Clampett was second after a 65.