3" is a big pipe to drive! I have one that drives 2 3/8. Works like a champ. You can rent them at a lot of commercial equipment rental places. They are vibratory and extremelly loud!!!!!!! Mine is an impact and hits the post 60-70 hits per minute. It takes about 2 minutes to drive a post 4 feet int eh ground! Greatest things ever made!
If its air power , how big a compressor u need to run it?
90-100 psi, 3 cfm
Man Saver Post Driver Looks like one that will drive 3" would require 90-100 psi and 3.5 cfm.
Unlike vibratory that require huge air supplies, these don't take that much. I bought the 2 3/8 version with a t-post adapter. It sure beats digging , hauling concrete, mixing concrete, leveling and waiting

Expensive, but my fence would have cost me as much or more in concrete. Not to mention my back moving the concrete! I also have no doubt I could get most of my money back if I want to sell it.