if you mean greatest common factor then that means
factor each and group the ones that are same
81=3 times 3 times 3 times 3
108=2 times 2 times 3 times 3 times 3
117=3 times 3 times 13
so we look for the group common to all (I bolded it)
81=3 times 3 times 3 times 3
108=2 times 2 times 3 times 3 times 3
117=3 times 3 times 13
the answeis 3 times 3=9
greates common denomenator of these nubers is 9