Turning a 4 channel into a 3 channel is the harder of the two scenarios. You need the reluctor ring that goes on the differential adjacent to the ring gear. It's my understanding that GM has discontinued this ring. Then you need to drill the hole for the ABS sensor in the top of the housing....UGH. Probably better off looking for a used 3 channel rear and sell yours.
FYI, 3 channel is ABS only
4 channel is ABS and TC.