Looks like you need to bond the respective pairs of interfaces, so:
bond0 = eth0 + eth1
bond1 = eth2 + eth3
bond2 = eth4 + eth5
There are lots of references on setting up bonding, and you'll need to specify to use load balancing (there are a couple of options, eg. round-robin) rather than active-backup.
If you are goign to use the server to route, then you'll need to enable the ip_forward parameter (again, google is your friend!).