Austin -- who had fired a final round 5-under-par 67 -- took home his fourth career victory on the PGA Tour after Summerhays and Beckman staged late charges to catch him at 20-under-par 268.
Summerhays got into the playoff thanks to birdies on the final two holes, rallying for a round of 3-under 69.
Beckman, meanwhile, also secured a piece of the 72-hole lead thanks to a clutch birdie on No. 18, completing a round of 5-under 67.
But neither could break par on the playoff hole.
Claiming sole possession of fourth place at minus-18 was Nicholas Thompson (71).
Billy Andrade (65) and Kyle Reifers (70) finished one shot farther back at 17-under 271.