combinations

start with triple of first character
AAA
increase last by one
AAA
AAB
AAC
et cetera
then when
AAZ
increase second by one each time and reset last, so
ABA
ABB
ABC
ABD
et cetera
when last is up again
ABZ
increase second by one again..
ACA
ACB
et cetera
when the second is up to Z (id est, the maximum character)
AZA
add one to the first one and reset the second!
BAA
BAB
repeat the steps above until
ZZZ
at this point the steps are complete.

*capital letters count as regular letters, just add them to the list after the lowercase or vice versa

-- KEY --
if for some reason you don't understand the terms used, here are what they mean
et cetera = and so on (same as etc)
vice versa = and the other way around
id est = that is to say