Sounds like you just want to set up bridging on the LAN interfaces. Why? Is this 2621 doing anything else, like WAN routing? Just trying to imagine what the problem is that you're trying to solve by using the 2621 in this manner.
You would probably be better off connecting the 3Com switches to the Cisco switches directly, and then just patching one of the router's LAN interfaces into the Cisco switches. Because the CPU on the router won't let you get full wire-speed routing. Probably won't let you bridge all that fast, either.
Connecting the 3Com switches to the Cisco switches will get you wire-speed bridging of those two LANs, if performance is a concern. And even if it's not.
If you have Layer 3 routing needs, a Layer 3 switch can do it much faster than your 2621; but some won't let you do access control lists, so if you need those then you want your router, or a Layer 3 switch that can handle them.
If you are looking for extra bandwidth along the lines of Fast EtherChannel, I don't think the 2621 can do this; and it wouldn't buy you anything anyway in terms of performance, only in redundant connections.
If you have two VLANs, or the Cisco-side users are one IP subnet and the 3Com-side users are another IP subnet, then it makes sense to put the router between the two. Each interface would get its own IP address, appropriate to the subnet it's plugged into. And the address on each interface could be the default gateway for users on that particular network.
If you have multiple VLANs, you CAN plug one 2621 interface into a VLAN trunk port (ISL or 802.1Q on the Cisco side, or just 802.1Q on the 3Com) and have it route among your VLANs. You need a "Plus" version of the Cisco IOS on your router, which may also require a Flash memory upgrade to hold the new image and more DRAM memory to run it.
I seem to recall that the 2621 has a max. forwarding rate of 37kpps or so; switches are capable of putting out 148.8kpps per port on a 10/100 interface running at 100 full duplex, so putting the 2621 in the middle of an important high-speed connection could make it the weakest link.
It is possible to link 3Com and Cisco switches into a cohesive network. Even with multiple VLANs involved. (I did this for another hospital system, with a Cisco MAN that used 6500's in the core and 3500's for distribution, and 3Com switches in each building for access.) But that's an issue better addressed in the LAN Switching and Routing forum here.
Hope this helps.
