Two internet connections and a third for failover

What is the best way to set this up?

I currently have two bundled t1 connections plugged in and working on my 220 firewall. One has my external IP addresses on it and the other is outbound only.

I recently got a wireless backup device that just needs to be plugged in and sitting there waiting to be used if the other two go down. Those two are from the same company so if one goes both will.

Those connections are set up on seperate interfaces and I am using multipath uplink balancing. The way I am reading it if I set things to failover, it will only use the first one and not the second. I still want to load balance over the first two and failover to the third.

I look at the bridging setup and am not sure if that will just delete everything I have set up on the individual interfaces when it creates the bridge. Will the external ip's still work? I have a number of servers with additional interfaces setup and dnated to internal servers.

Is there a way I can set this up without deleting the interfaces and starting over? We are a 24/7 business and I can't expect much downtime to do something like that.