3 simbols all variables added in listbox

Question

Hello, I have problem that I need to solve.
I have 3 string "a", "b" and "c".
I need all varabiles to be added in listbox.
Sample of list:
aaa
aab
aac
abc
abb
acc
acb
baa
bba
bca
caa
cba
cca
cac
bac

etc.

Can someone show me some code for this problem ?