Hmm - those 3Coms (at least their ISA brothers) had a DOS tool coming with them for configuration (media, I/O port, IRQ, ...). Try setting everything to default in there, possibly something's off.
If a basic ping doesn't work you can try this:
From a Windoze PC you can check if TCP/IP's is basically running by issuing
ping <Amiga's IP address>
arp -a(need to be input rather quickly since ARP cache times out in 10 secs or so)
In Linux it's something like arp -i AFAIR.
In comparison to a normal ping this also works with 'stealth mode' (ICMP Echo request not honored) and with borked up routing. The only thing required is that the target NIC's got an IP address and sender and target must share the same subnet which can be accomplished by temporarily setting up the sender with a netmask of 0.0.0.0.
