3 NICs one router, setting wireless as primary

I work for some doctors that all have MAC, seldom do I need to work on them, when I do I have always been able to solve any problem I have come across, until now.
in one MAC Desktop I have been asked to install Parallel's and install Windows XP as the VM. I got that all done very simple to complete. Next I setup one NIC as the Paralle's NIC and one as the System's NIC. Everything is working perfectly . . . Now they wanted access to an unfiltered wireless connection to by-pass the firewall. I installed a simple USB wireless NIC configured it and got it to connected status. I have pointed all three NICs to the wireless gateway. I can ping facebook but can not access it via the browser, I assume this is a DNS issue. If I turn off the onboard etho, eth1 it works fine. Any ideas of how to set priority of NIC used. Or how to flush DNS on MAC . . . ?

Sounds more like a routing problem to me. I'd do a "netstat -r" in the Terminal or the "netstat" tab in the Network Utility and see if you have multiple default routes. If the wireless connection is the only path to the Internet then that's the only one that should have a default gateway. You may have to statically configure the other two interfaces to ge this to work reliably.