Is it possible to set a personal network up between my computer (3G modem) and cell phone (3G modem)? If so, what software would I need to be running on each? Could I just manually set the attributes?
No you can't, not via 3G, and if you did it would probably be illegal as the frequency bands used for 3G are most likely licensed to Telco's, and not for personal use.
However, if your cellphone supports wifi, then you can set up an ad-hoc connection with it and the computer. If your phone can't do ad-hoc mode, then you can install software such as Connectify to turn your computer into a wireless hotspot, just like a wireless router.