If I understand yor post correctly, you want to take your existing 3" exhaust hole and enlarge them to 4". If that's the case, you can take your large hole saw mandrel and screw on the 4" hole saw. Then scew on the 3" inside the 4". The 3" will stick out just enough to act as a pilot for the 4". I tried to attach a picture, but I don't think it came out to good.