We used to have a fractional Internet system based on the number of phone lines. There are a few ways to split it using routers alone or with switches. But, and I may be wrong here, but you may want to use a DMZ or VLAN to secure it from the office you are sharing it with.
Personally, it just seems easiest to allow the company supplying the T3 to set it up for you. They could channelize it.
