Is it possible to access an iPhone over SSH on 3G?

a) Unless you have jailbroken your device, iOS doesn't have an SSH server to run.

b) If you have jailbroken your device, it will not be possible to SSH into your iPhone over cellular. You are behind one of the most complex NAT setups there is. When you move between towers, and lose signal, you lose your lease to a unique ID with the cellular network (like an IP). However, every time your connection is re-established with your carrier, you're connected to a new tower and your connection is routed differently through another NAT and finally out somewhere, probably in another state.

In researching this alone, over the course of 10 minutes, my iPhone returned 2 different IPs from the internet, one in Florida, one in Pennsylvania.

Short answer: Wi-Fi = yes | Cellular = no

Yes, you can, if you use a VPN service, that assigns you a real, unmasked IP address (which will be accessible from the internet) or a VPN to your local network.