Grande Shores shot through an opening along the rail on the turn and drew off down the stretch to get the third stakes win of his career Saturday, beating Squadron A by 1 3/4 lengths in the $100,000 Sir Shackleton Stakes at Gulfstream Park.   Grande Shores, an 8-year-old son of Black Mambo, is a homebred for Fred Brei’s Jacks or Better Farm and is trained by Stanley Gold. The horse has won 15 of 54 career starts for $587,649. He raced for as low as an $8,000 claiming price back in 2013. Prior to Saturday, Grande Shores had made four consecutive starts in graded stakes at Gulfstream Park over the winter, finishing second in the Grade 3 Mr. Prospector and third in both the Grade 3 Hal’s Hope and the Grade 3 Fred W. Hooper. His previous stakes wins came in the $110,000 Claiming Crown Rapid Transit in December 2014 and the $60,000 Parrot Key last June. Defer Heaven, the 6-5 favorite, set the pace in the Sir Shackleton, covering a quarter-mile in 22.62 seconds and a half-mile in 44.86 while pressured by W V Jetsetter. With Edgard Zayas aboard, Grande Shores ($9.40) took advantage of the opening along the inside and surged clear, leading by 4 1/2 lengths at the head of the lane and finishing the seven-furlong race on a “good” track in 1:22.96. Squadron A finished 6 1/4 lengths clear of third-place Brothersofthetime. They were followed by Just Call Kenny, Defer Heaven, and W V Jetsetter in the field of 4-year-olds and up. Anchor Down, Yourdreamsormine, and Griff were scratched.