Hi,
You may need to set up port forwarding for each PS3.... I know we dont have any problems with the multiple Xbox360's here with voice chat.....
I am assuming that if you two are playing alone...then adding a player would still keep you as game host(s)....so if I am correct, then there is probably some setting getting wonky about your hosting thru the DIR...again, if the PS3 is similiar to the 360 in that process....
SD1