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Ryan Farquhar took victory in the Open race, run on Friday night, the classic race and the Moto 450 outing.
William produced a dominant ride in the main event of the meeting, taking the chequered flag by 28 seconds from runner-up Jamie Hamilton.
Mullingar's Derek McGee was third on the Wilson Craig 600cc Honda.
Earlier, William followed up his double win in the 600cc category at Cookstown by winning the Supersport class from McGee and Hamilton in difficult conditions.
Weather conditions were better on Friday evening when Farquhar won a thrilling Open race by 0.4 seconds from Tyco BMW pilot Dunlop.
Early leader Hamilton, from Ballyclare, completed the podium positions.
McGee took the flag in the Supertwins class, seeing off the challenge of Hamilton.
Jeremy McWilliams, making his debut over the 5.3-mile circuit, was third in the Classic race.
William Dunlop clinched a double success at the Tandragee 100 meeting, winning the feature Superbike race and the Supersport 600cc event.