Auto generate customer number [ref. must be from 2 tables]

Question

Once again I needed your precious help.

 

I have 3 tables in sql server 2014 namely: tblBranch, tblAgents & tblCustomers. I need to have an auto generated customer number with format numbers: xx yyyy zzz, where xx is the branch code(string type), yyyy is the agent code (string type) and zzz is the customer series number also in string type. Below are my tables.