Thornton aced the par-3 11th home at breezy Aa Saint Omer Golf Club and finished in three-way tie for first place with local favorite Baptiste Chapellan and second-round co-leader Tjaart van der Walt.
Van der Walt had been ahead by three shots on the back nine Saturday but was undone by a double bogey on the 16th holed and bogey on 17, which allowed Thornton and Chapellan to elbow their way into first place.
Thornton carded a 6-under 65 for the round and was 4-under 209 heading into Sunday's final tour around the links. He nearly tied the course record of 64 thanks to his ace on the 206-yard 11th, which came courtesy of his seven iron.
"I was in between a seven iron and an eight iron and I hit a nice smooth seven and it was all over the flag, one big bounce and it just rolled in," said Thornton. "It was really good shot."