Inside diameter doesn't mean anything here. Pipe size is measured from the outside. Cast iron can have different wall thicknesses, so you must use the actual measurement. You need a special banded coupler to go from the old cast iron to 3" PVC pipe. Then your 3" PVC tee will connect to that. By special, I mean one that will fit your cast iron on one end and 3" PVC on the other. You won't find it at a Big Box store. You need to go to a real plumbing supply house...one where pros buy their materials.