UPDATE 2-Golf-Spieth wins Valspar after playoff thriller

(Adds detail) March 15 (Reuters) - American Jordan Spieth won the Valspar Championship in dramatic fashion on Sunday, triumphing in a three-way playoff, with a superb 28-foot birdie putt on the third playoff hole. Spieth outlasted fellow Americans Patrick Reed and Sean O'Hair at the Innisbrook Resort in Palm Harbor, Florida, after all three had finished 10 under-par for the tournament. The victory is the second of the 21-year-old Spieth's career on the PGA Tour following his win at the John Deer Classic in July 2013 and will move him to sixth in the world rankings. With no difference between the trio on the 18th and 16th holes, the contest went to the par-three 17th, where Reed, who found the bunker, and O'Hair left themselves with par-putts before Spieth drained his birdie putt. "Putts like that are luck, right?" said Spieth. "If that doesn't hit the hole, it's four feet past. I guess it was just my day." It was a cruel ending for Reed and O'Hair who both had played some excellent golf. Reed's bogey free, five-under par round of 66, featured some outstanding play around the green. He kept himself in the playoff fight despite finding a bunker on the 18th and then deep rough at the back of the green on the 16th. O'Hair was close to glory on the second playoff hole where he saw a birdie putt, which would have won him his first PGA Tour event since 2011, lip out. Rookie of the Year in 2005, O'Hair has had a career in free fall as he has slumped to 401st in the world rankings, but the 32-year-old's showing this week indicated he could have finally found the confidence to get himself back on track. Spieth only made it into the playoff thanks to another great putt, sinking an 11-footer on the 18th hole after he had made birdies on the 13th and 14th holes to get in the frame. Overnight leader Ryan Moore, had opened up a two-shot lead with an eagle on the par-four sixth, where he found the hole from 172 yards with a seven iron. But Moore fell apart down the stretch and bogeys on the 16th and 18th cost him a place in the playoff and left him two strokes off the leaders. (Reporting by Simon Evans, editing by Gene Cherry)