Setup AlwaysOn with 2 Secondaries and not adding all the databases for 2nd Secondary
You cannot exclude databases from replicas in an availability group.  There may be a way to accomplish this with a hack, but it would be an unsupported configuration.

In order to accomplish what you are specifying, you need two availability groups.  Availability group A would have all three servers as replicas, and would contain only the databases that you want to be replicated to all three servers.

Availability group B would have two servers as replicas, and would contain the one database that you want to be replicated to only those two servers.