Unicode, I would be leery of 3 way calling through a LEC. Generally speaking you get billed for every single 3 way you do on top the phone call. XO just uses T1 to "long haul" dialtone to your channel bank. It should act just like a POTS lines (possibly hotter voltage-wise). I have MANY customers that do, what you are trying to, do with just a few Centrex lines.
