The letters are used are a,b and c
A - aaa
B - aab
C - aac
D - aba
E - abb
F - abc
G - aca
H - acb
I - acc
J - baa
K - bab
L - bac
M - bba
N - bbb
O - bbc
P - bca
Q - bcb
R - bcc
S - caa
T - cab
U - cac
V - cba
W - cbb
X - cbc
Y - cca
Z - ccb
