copy a file from a third pc

I've got a PC that is on the same intranet as I am, and I'm local admin on
both machines. The second machine has a second nic in it and it is attached
to a third pc that is not on the intranet, and I need to get a file from
that third PC. I know I could start a remote desktop session with the
second and get into the admin share on the third pc, but option isn't always
avaliable.

So the PC I'm using is PC1 and my IP is 131.192.0.1
The second is PC2 and it's IP addreess is 131.192.0.2 so \\pc2\c$ is fine

PC2 has a second nic in it with a crossover cable going to a stand alone
pc - PC3
The second nic in PC2 is192.168.100.101 and PC3 is 192.168.100.100 - From
pc2 I can do a \\PC3\c$ with no problems

What do I need to do, so I can get a file or open PC3 while I'm on PC1 and
move it to my HD?? Any suggestions