This can easily be done in hana using case statement .
case ( leftstr ( "YOUR_FIELD",1 ) ,'A','AAA' , 'B' ,'BBB' , 'C', 'CCC', 'XXX' )
it will take first character of your field and if that is A ----> AAA
B------> BBB
C------->CCC
For all other values -----> XXX
Check run time before doing this, and compare it after defining this in HANA model. I think hardly it would make any difference in performance.
Cheers
Anindya