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Question on HA configuration.

Dear Sir,

As you have mentioned earlier below. 

wtprd0X (search for ‘wtprd0’) - these are front end web server nodes for an HA cluster (uses HAProxy in pfSense)
wtsqlprd01-3 (search for wtsql) - these are 3 MySQL servers for the backend of a site in HA

I can understand the purpose of 3 frontends but how 3 Mysqls are designed are they replicating to each other get updated or or is this a enterprise cluster.
Cluster in a sense where you are running multple nodes of MYSQL but application sees only one and if any of the instance goes down the other stands for backup. is it like.

 how does the 3 MYsql stack works. ?