Once you install OpenWrt, you can actually collapse the network down to a single router (or at least a single one of those devices doing active routing)... if you need additional APs, you can use the other 2 units in the dumb AP configuration.
Meanwhile, you'll use mwan3 to setup the multiple wans, and either that or policy based routing achieve the specific lan > wan mappings you desire.
Thank you. I think mwan3 will do the fail over. I am not sure how to mark traffic from mac address A on network1 to tell the last router with mwan3 + internet modems to specially do an exception routing. E.g. Xbox on network 1 will always through telecom since it has lower pings and not use cable modem.