If I have a hole that's 3" 1/2 can I increase it to 3" 3/4? What do I need to use to acomplish this?

I realize that hole saws require a pilot bit that keeps the saw steady. So just going at it with a 3 3/4ths hole saw might make a mess of things.

Thanks,
James

Don't even try to use a hole saw.

Mark the circumference of the hole with a marker.
Use a 14-16 tpi (metal cutting) blade in a jig or sabre saw and cut away.

Use care.

a dremel with a grout removal bit, high speed bit, or similar bit style work wonders with the above mentioned line method.