You may wish to consider something in the PIX or ASA line.
If you have an inside router with VLAN support, then you could use as little as a PIX 506E, running the third interface as an 802.1Q VLAN on the inside physical interface. If you need three physical interfaces, the PIX-515E-R-DMZ-BUN can apparently be had for as little as $US2150 (Network Warehouse) though most places are ~$US2500.
The corresponding ASA model with multiple physical interfaces (that are not switched together) is the ASA5510 -- though it appears that the ASA 5505 with the Security Plus license could also do it. THe part numbers look to be ASA5505-SEC-BUN-K9 and and ASA5510-BUN-K9; looks like you can get the 5505-SEC for less than $US1100.