My father needs to replace an existing 3" copper bathroom drain with new 3" plastic... How does connect the existing 3" copper with the new plastic?

Hi DD...

You would use a SHIELDED CLAMP to connect the two different pipes (see 1st picture below). Specifically, you want a 3" copper x 3" plastic (PVC or ABS) shielded clamp. Trade names for these clamps are the mission clamp or the proflex clamp. These can be found at home supply stores sometimes, but can always be found at the nearest PLUMBING SUPPLY HOUSE... ;)

Do not use the all rubber with two SS clamps (2nd picture). These are illegal for use above ground.

Good luck!

MARK

3 bye 3 frinco at any store copper to plastic