considering the point of a "single legal(???) IP" to be assigned, seems there is only one point of internet access. this is where the isdn router resides. i suppose you are thinking of connecting the three buildings with their three seperate seperate networks to this one.
use IP subnetting on the building networks, create three Class C subnets to accomodate all your network nodes i.e nodes, printers, servers etc. this would avoid network traffic bleeding into the other building, and a light sense of privacy as well.
if you are trying to avoid data being not broadcasted to every host under a single network, use switches on the other side of the isdn routers. each building has a switch, hence broadcast storms, or redundant data traffic can be avoided.
Picking pebbles on the shore of the networking ocean
