MID function not working properly

I wonder if i missed something, but

with 2 indicates starting point is "b" (1 is "a", 2 is "b", 3 is "c",...)

and there are 2 skipping columns between 2 cells

try in first cell then drag accross:

=LEFT(LOWER(ADDRESS(1,(COLUMNS($A:A)-1)*3+2,4)),LEN(ADDRESS(1,(COLUMNS($A:A)-1)*3+2,4))-1)