You won't have to do anything special when you talk to the apprentices. You'll just need to hand over a
Notice of Termination, and they'll depart peacefully. Then once you've delivered the bad news to all three apprentices, when you return to any of the Mages' Collective liaisons (in
Denerim Market District, or the
Lake Calenhad Docks), you'll receive 1 sovereign and 175 xp.