Yes, that is a 3" CGA 320. That's what I have used. Has a built in filter and all.
As Jim mentioned, it is hard to remove those industrial cga nipples. Victor uses lock-tite on their connections to insure they do not back out, etc.
I use air tools, which makes simple work of them. Also as Jim suggested, once you remove the nipple, you'll have to clean out the port. You can used compressed air (either from an air compressor or one of those canned contraptions for cleaning out your computer keyboard). I've also used q-tips w/ alcohol on them.
Before installing the new nipple, wrap the threads with several layers of teflon tape. Tighten the nipple as tight as it will go. Then, you can connect the regulator to a co2 tank (closed obviously) and rotate the whole regulator to torque the nipple down that much more (gives you more leverage if you don't have a vice).
So a vice is helpful. If you don't have a vice, air tools can save the day. If you're still uncomfortable, you can always send the unit to me, and I will do the dirty work for you.
a simple adjustable wrench works to tighten the cga nipple. easy to control and won't round the nipple.