I'm making the following assumptions
1) There are 3 spaces for the letters but we can have repeats
2) There are "N" blank spaces
So, the total number of spaces is N + 3
And we want to fill 3 of the spaces with a letter (allowing repeats)......so we have 33 = 27 ways of choosing the letters.
And the number of arrangements of these blank spaces and letters is just C(n+3, 3),
So the total possibllities are 27*C(n+3, 3)
