You need to post a description of your hardware, i.e., whether you have a router, bridge, etc. and your net topology before we can help you with this one!

............+-----[Router C]----[Internet]
............|
[ LAN 1] ---+-----[Router A]----[LAN 2]
............|
............+-----[Router B]----[LAN 3]


Router A & B can be an NT 4 or 2000 server each with at least 2 NICs. You will have to turn on routing between the cards under Networking.

A simpler and less costly way would be if you purchased some very generic and cheap routers.

An even simpler would be if you just forgot about the stupid routers, got a decent switch, created three VLANs in the switch and went home and had a rhubarb pie. This scenario would look like this:



[LAN 1]---+|------|
[LAN 2]---+|Switch|---[Router]---[ Internet ]
[LAN 3]---+|------|


In essence, the switch allows you to create three, separate, and independent virtual LANs, or VLANs within a single box. You can designate certain ports on the switch as to belonging to what VLAN, and make your life a whole lot easier. You could further expand this to include bazillion VLANs without adding any new hardware (but then you wouldn't have time to eat pie, right?!)

Now go and have some pie.