I have a 2 bathroom they ran a vent off 3in pipe is that needed looking to replace coppper with pvc and move it 4 in into wall

Can you show us some pics of what you have?

More details please! A picture would be helpful also. Are these bathrooms back to back and was the 3" vent to service both bathrooms? If they weren't vented before then ,yes, the vent was needed. Regards, Tom