"My plan was to cut a section of the current 3 inch ABS out and leave room to attach a small stub pipe to one end of the WYE. Then slide the Fernco fitting over the other end of the existing pipe, insert in the WYE with the stub piece so there is about a 1/4 gap between the stub piece and the existing pipe and then slide the fernco over this 1/4 inch gap and tighten the clamps after I glue the WYE to the other existing piece."
Sounds good to me with one exception. I would change from a Fernco Neoprene Coupling to a more ridged NO-Hub Band, Neoprene couplings are not ridged and will sag in time leaving a dip in the line that could cause a clog. Good luck, Tom