Cannot install SQL Server on a local disk
If I understand this correctly, you have 2 servers configured for in a FCI installation. They use shared disk which is the Y drive. You have a 3rd server that has local disk which is the Y drive. 


Node 1 and 2 should be installed first as a FCI instance. 
As long as the FCI is working as expected and fail overs are happening as expected between Node 1 and Node 2, Node 3 should be added to the Windows Cluster as an extra node. 
Node 3 should have SQL server installed on it as a standalone node. 
Enable Always On feature on Node 3 and the FCI instance
Node 3 should not be listed as a possible owner of FCI instance. 


You should then be able to have an FCI install and a standalone install on the same windows cluster. You will be able to automatic failover between Node 1 and Node 2, with manual failover to Node 3.

I have this set up in my lab now.