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