Highlight groups of data the nth row

I have a list of data containing several thousand rows.
I need to higlight every 3rd 'letter' so C, F, I etc . The problem is that data contains groups of random sizes. Here is an example below

A 1
B 1
B 2
B 3
C 1
C 2
D 1

I need to be able to change the colour (background or text) on every 3rd character as well as rest of the group ie C 1 &2

Hope you can help