SD-WAN with layer 2 multipoint metro ethernet

I'm an sd-wan newbie so bear with me. We have 3 sites with Fortigates that each have their own internet connection as well as a metro ethernet connection that basically acts like a virtual switch meaning that they all have a layer 2 connection to each other over a single ethernet interface at each site. I set them up on the same subnet and they can all reach each other and pass traffic. I have ipsec tunnels that are all working as well.

I've messed around with a simple link monitor before, but I'm wondering how I can setup sd-wan to automatically fail over the connection between sites. It would be nice to have the monitoring that sd-wan provides on the quality of the link. When I try to add the metro ethernet connection, it needs a gateway and I'm not sure what to put here since there are multiple sites it can reach and there is no ISP gateway. Is there a way to specify a different gateway based on the destination subnet? Any ideas or links to tutorials. I've done a ton of searching but can't find any simple answers.