To an extent they are compatable. I have done some work on this before and there are some tricks. For example. OSPF over frame relay links - you need to statically add the Cisco's address into the 3Com's arp table. My advice would be to test your implementations before doing it in a live environment. I have come across other routing issues as well. Based on the fact that in a Cisco world a static route is prefered and in the 3Com world a dynamic router is prefered. So there will clearly be issues related to this.
