Is there a provider who can terminate 3G video calls, and convert them to SIP video calls so that they can work with Asterisk.
Otherwise is there a gateway that you buy (i googled, and found a product from radvision), but am not sure who supplies the 3G PRI? - not even sure it is called a PRI.
Where are you located ? You would have to find a carrier in your area. Not sure if asterisk can support the video from a PRI. If you can get a gateway that can convert it to SIP that may work.
There is a project to add 3G support in Asterisk, some people had success with it, using PRI lines, look at lists.digium.com/pipermail/asterisk-video/ , should be interesting for you.