AG of 2 replicas on 2 nodes in WSFC : is it possible to add 3rd replica to AG and NOT add 3rd machine WSFC?
Is it possible to add a 3rd replica (SQL Server 2017 on Windows Server 2016) to existing availability group, without adding a 3rd machine to existing WSFC ?


No, there are explicit checks to see if the node of an AG is already hosting the same AG. If you want a 3rd replica, add a 3rd note.

I'd also advise against stacking up on multiple instances of SQL Server on the same machine, simply for administrative and troubleshooting purposes.