First of all you have to find out which card gets which interface from (eth0 to eth2). Assuming eth0 is the 1st card, eth1 the second and eth2 the third one, you will need something like this
iface eth0 inet static
iface eth1 inet static
iface eth2 inet static
(change the gateways as you need)
Plus, you will have to remove network manager or wicd so as they won't interfere with those.
And check /etc/udev/rules.d/70-persistent-net.rules to find out which device gets which interface.