Hubs and Bridges?

There are a couple of pieces of information that would be needed for a complete answer. First of all, it makes a difference what network protocol you will be running (IP, IPX, NetBeui?) and how much traffic will be going from one LAN to the next.
If you are running IP and each LAN must be on it's own subnet, then you really need to route rather than bridge. At the cheap end, you could build a three-interface router using PC hardware and Linux Router Project software and probably put the whole thing together for $100. A commercial router with three Ethernet interfaces should probably be around $5k but at the moment, I can't think of one that comes with 3 interfaces out of the box except for the high-end modular type (e.g. Cisco 3640).

On the other hand, if you could combine all the LANs onto one subnet, you could just use a network switch to join them together. I have a D-Link DSS-8+ 8-port FastEthernet switch that has performed excellently in a similar application and it cost around $150 if I recall correctly.

paul