SQL Server 2016 Availability Group - 2 Nodes as Primary & Readable Secondary, 1 Node as DR
I would set up your configuration as follows...

Node and File Share Majority quorum model

Node 1, Node 2 and file share witness at primary dc

Node 3 at DR dc

1 vote to each node in the primary data center &amp; fileshare witness

0 vote to node in the DR data center

preferred owners node 1 &amp; node 2

AG synchronous mode between node 1 &amp; 2 automatic failover

AG asynchronous mode to node 3 manual failover

primary AG on node 1 with readable secondary replicas at node 2. Since you plan to use readable secondary replica, you will have to license node 2 as well as node 1. Node 3 will be covered by software assurance. I assume this is a production environment.