No. it is not possible to have more than two PMP servers for HA solution. To elaborate more, High availability and cluster are two different replication mechanism we support. So, for replication with SQL database, PMP comes with below features.
1) High Availability This would have two separate application server and two separate database server and the replication would happen between these SQL databases. This requires the replication components to be installed on MSSQL database server. Refer the link below for more info on this method. Here, we use the native SQL Merge replication mechanism.
With High availability, Primary server provides full access while the secondary server will offer 'Read/Write' access to the users (except password reset), until the fully-functional primary server is brought back to service.
In this case, you can very well have the application servers on two different geographical locations with a load balancer in front. We do have customer have application servers in two different location with F5 load balancer.
2)MSSQL Cluster with Always ON nodes with FOS - We do have an option to use MSSQL Cluster with Always ON nodes with FOS - Fail Over Service: The idea behind this set up is, Master1 server will be communicating to the back end MSSQL DB in real time and the Master2(FOS) will be monitoring the Master1 PMP web server, in the event of Master1 web server being unavailable, Master2(FOS) will start the PMP web service in the other server and it will automatically take over the DB connection and will act as a new Master PMP web server. Once the Master1 server starts, it will automatically convert as a secondary and start to monitor the current Master2 web server which is actually communicating to the DB.
Here, the failover will happen using the PMP application. Also, both the application server should be in the same subnet