I typically use one bridge per network card. I've never thought about assigning all nics to one bridge, but I guess it can work if you managed it. You typically only need one nic to connect to the wan, and one to the lan. Eth2 can certainly have an IP address that's in the same range as your eth1 card. One address can be used as the gateway and the other can be used for file sharing to keep the traffic apart, for example. I'm not sure what you use your centos box for, but it's possible you don't need an eth2 at all...