Replacing Vent Pipe

I have a house built in the 60s that has a copper 3 inch main vent. The pipe is corroded most of the way up and is leaking small amounts of sewer gas. I want to replace the pipe, but want to use ABS or PVC. The problem is that the wall is 3 1/2" thick (2x4 studs) and the OD on plastic is bigger than copper. On top of this, the tub and sink vents connect in the wall. A typical hub on a plastic connector is 4" which does not fit in the existing wall space. Any thoughts on solutions so I don't have to rebuild the wall? Thanks

You could run 2 vents. The first one would be a your 3" vent from the floor below,just run it straight up to the attic with no joints in the wall-thats one reason why you can get 10' lengths. The tub and sink vent can be connected together above the flood level rim of the sink as an 1 1/2" and then connect to the 3" in the attic.