I have a 3" metal drain pipe in the wall (my Dad installed) for a wash machine the inner diameter is a female thread measuring 2-1/4" . I would like to attach a male threaded PVC pipe into this but all I found are 2" and 2 -1/2" pvc male threaded pipe. I do not know the outer dimension of the threads of a 2-1/2" pvc pipe. Is it 2-1/2 or 2-1/4 .

If you have a female thread then it's a 2" galvanized coupling. Try a 2" PVC male adapter with 3 turns of Teflon Tape wrapped around the threads. Good luck, Tom