Multi-Subnet Always On with Availability Group for DR
Usually, if you want to use a node for "failover" (high availability), you want to have the AG set for synchronous replication (to enable the auto-failover).

That being said, if you set it to "sync" and it's in another site, you may have performance issue caused by network latency.

Also, if your "read only" node is not part of the AG, then you will have to implement something else to keep the data sync (or close to sync)

This is why we usually see 2 nodes on the same site (for High Availability) and a 3rd node on a remote site (for DR) that usually is an async replica.

I hope it will help you to make the right design.