Donaldson splashed home in 69 for a three-round score of 12-under 204 and a narrow lead over Anthony Wall, who had an impressive round of 67 at Royal Portrush.
Donaldson, who carded a 67 Friday, bogeyed Saturday's first hole but atoned with an eagle on No. 2 and finished the rollercoaster round with three birdies and another bogey before making the turn and finishing at even par on the back nine.
Second-round leader Gregory Bourdy stumbled back to 4-under for the tournament after slogging to an 8-over 80 on Saturday.
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