Hi Mike,
I would suggest the Cisco 2821 ISR router for you. It has a rated forwarding performance of up to 87 Mbps, so it should be able to handle a full DS3. It has 1 network module slot and 4 interface card slots. So you should be able to get:
2 x WIC-1DSU-T1-V2 (1-port T1/Fractional-T1 DSU/CSU WIC)
1 x NM-1T3/E3 (1-port clear-channel T3/E3 network module)
OR
1 x NM-1A-T3 (1-port DS-3 ATM network module)
If you are expecting your traffic requirements to increase in the near future, you might want to consider the 3825 ISR router.
Hope that helps - pls rate the post if it does.
Regards,
Paresh.