Setup AlwaysOn with 2 Secondaries and not adding all the databases for 2nd Secondary I have a situation where we have AlwaysOn Setup in SQL Server 2016 Enterprise editions with primary in region A and secondary in region B setup in Asynchronous-Commit mode. We have 4 databases in participating in this AG. Now we would like to add all another secondary in same cluster in Region A to different server. Out of 4 databases we only want to add 3 databases to 2nd secondary which will be in region A. Last database is heavy write application which grows quickly and we do not want to sync it between 2 secondaries. Is it possible to achieve and is there any performance effect of it?
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.