I am trying to repair a toilet drain that is existing 3 inch copper. I am changing my toilet drain to a 3 inch pvc going down to a 3 inch 90 degree pvc fitting.out of this fitting I have a 4 inch long piece of pvc and want to use a 3 inch fernco [name brand] plumb quik fitting. The fitting is a rubber fitting with steel bands. It fits quite snug on the pvc but quite loose on the copper.the existing copper has a smaller outer diameter. Will this work.

You can purchase Shielded Coupling that is : 3" PVC to 3" Copper. It will fit exactly as it should.

However, you could also tighten the band on Fernco fitting over the copper - even it is not as tight as it is over PVC.