(mysql) Database replication between 2 DB while 3rd DB as backup I am not an advanced DB admin or designer. I am sure there are articles on this kind of scenario, probably too many. So I got confused and not sure which solution is appropriate/best for me.

I am setting up two DB on different region. Users will access/write to the nearest one. So these 2 DB should be 100% identical. Any insert, update, delete on a DB replicate same on the other instantly.
(better if table structure on a DB can also alter other DB table)

See the attached diagram. Clients can use any of these DBs.

There is the 3rd DB. Clients won't ever access it or write to it. 
It's for backup purposes only.
Any changes but "delete" from other DB will be replicate to this one.

Any suggestion?
Dual-Master, where you write to both masters, is full of problems.  You need to carefully set the two auto_increment_* "variables".  You need to make sure users cannot insert the same PRIMARY or UNIQUE values into different masters.  If 'East' goes down, the Slave is hard to reconnect.  If a hurricane hits the East, you lose both the master and slave there.  Etc.

A slight improvement is "Dual-Master, Single writer".  That is, you have the configuration you described, but clients never write to more than one of the Masters.  The configuration is more stable, but the failover process is perhaps more complex.

Instead, consider Galera (in MariaDB or PXC), which solves many of the problems.