AAG expansion between datacenters
You can absolutely add a 3rd node to a DR site and have it be an asynchronous replica. It'll be a multi-subnet configuration, so be sure you add the MultiSubnetFailover=True to the connection strings to get a successful connection, that is if the drivers you are using support it.

You may want to disable the 3rd node's quorum vote in your configuration. I'm assuming you already have quorum setup correctly with a fileshare witness or something else (if you don't, then get that fixed). You don't want a network glitch to take down the cluster if you also have an issue on one of the nodes at the primary site. Be sure you understand quorum and votes and what's needed to keep production up.