Hi Chandi,
Regarding your second question you must concatenate all 3 codes to make one 'external transaction code'.
For example
External transaction code would be: PMNTICHQCCHQ
Then you can link your posting rule to this
Regarding your 1st question, I would expect this to depend on the bank and how their statement engine works. I suggest discussing this with them and then figure out how to reassign your posting rules from the BAI codes to CAMT ISO codes.
Hope this helps,
Jack