Hi!
I think subtraction is better way. You have to subtract 100, 10 & 1 and count the number of possible operations. Please take a look at left side buttons. You can find "code library" and after choosing it examples of hex to decimal conversion.
Make your own led patterns table in rom memory. After converting hex to the decimal, use the result as index. Simply add the index to the beginning address of your table, take the pattern and output it.

Wish you good luck,
Jacek