communication problems

I have 3 computers that have a communications problem
pc 1:
Windows XP pro, connected to ADSL internet.
Norton Firewall.
2 Network cards
This pc have Internet Connection Sharing.

pc 2:
Windows 2000 server
member of a workgroup same as pc 1

pc 3:
Windows 2000 pro
member of a workgroup same as pc 1.

All the computers are connected to the same switch. pc 1 and pc 2 working fine, both in intranet and internet.
The problems starts here. pc 1 can communicate (ping) whith pc 2, butnot whith pc 3.

pc 2 can communicate whith both pc 3 and pc 1.

pc 3 can only communicate whith pc 2, not pc 1.
Can anyone explain what I can do to resolve this problems??

Roy F