Some additional notes, you might want to keep in mind.

As a rough rule of thumb, you want to size such that processor doesn't normally average above half to two thirds utilization. This to insure there is some reserve processing.

Don't forget all traffic needs to be routed, e.g. duplex. E.g. FastEthernet can deliver 200 Mbps that needs to be forwarded.

Generally, forwarding performance increases as packet size increases.

Actual forwarding performance load is impacted by configuration of various services. A non-basic configuration will generally require more CPU.

Cisco recommends a 3825 is suitable for a half T3; a 3845 for a full T3. These are conservative recommendations, in practice, you might find either 3800 can often handle 2x this recommendation.