Waste tank collar is corroded

What material is the tank made off? Since you said corrosion I assume it's a stainless steel tank. If so that fitting can probably welded but if there are other sign of corrosion it's probably better to replace depending on how much access you have

Fiberglass is best for holding tank, no corrosion, or poly.

Galvanized isn't the best for a holding tank. How old is it and what kind of boat?

Patching it up is fine if there are no sign of corrosion elsewhere but if the tank is rotting from the inside you re going to have to replace it sooner or later

Any metal for a black water tank usually proves to be the wrong material to use.

I believe it is galvanized. Hopefully not.
This will loose to the powers of the black tank.

What is the tank made of?
If it's a metal tank also, electrolysis has or will soon destroy everything.

Either way it will be a sh**** job.

Pictures? What boat?

3" pipe is pretty big for this application. Does the pipe come off the bottom of the tank and run horizontal under the tank? Can the pipe be removed and plugged at the bottom of the tank?

You might be able to drop a pipe down from the top of the tank to the bottom for pumping it out. You might be able to plug the existing exit from above, then install a suction pipe at the same time with that same access hole.

Working on or taking the tank out might be simple or impossible, no way to know without pictures and better descriptions of the problem.

Pictures...
Don't be lazy when you ask for advice: Shoot pictures and post with your question.
Then you may get better advice instead of folks guessing what you are talking about and giving general answers, rather than specific advice.
Easy.