Changing master for channel gives syntax error
You're looking at the documentation for MySQL 5.7, but you're running MariaDB 10.1, and replication channels happen to be a feature that is implemented differently in MariaDB.

From the MariaDB documentation on multi-source replication:


  You specify which master connection you want to work with by either specifying the connection name in the command or setting default_master_connection to the connection you want to work with.


So in your case you could try:

CHANGE MASTER 'master-203' TO MASTER_HOST='192.168.0.203', 
MASTER_USER='rep', 
MASTER_PORT=3306, 
MASTER_PASSWORD='pass', 
MASTER_LOG_FILE='mysql-bin.000003', 
MASTER_LOG_POS=107; 


(Disclaimer: I've not tried this in practice.)