ok here is my situation i have 3 computers that i can use for setting up servers all i will be doing is hosting a small website and email i would like to use one of the computers to set up a firewall and then one for email and the web site now my question is can i some how set up the 3rd computer as a back up and what os should i use for the firewall

This really depends on what type of firewall software you want to use. I don't know from personal experience but it seems to me a lot of DIY firewall junkies like to use LINUX on their dedicated firewall box so obviously there are some aplications for that OS. I am sure there are others (firewall apps) for Windows, etc.

So I suggest you settle on the firewall you want to use first and then install the OS on the third box accordingly. Again, sorry I can't be more specifically helpful. Maybe a firewall guru will drop in on this thread.