hello, generate all combinations of at most 3 by letters,

hello, generate all combinations of at most 3 by letters, we have 10 letters
* in 7-letter formations =
we have = ABCDEFGHIJ
A b c d e f g h i j
1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30

then = aaabcdef ............ ,, hhhijjj
maximum three per column

plaese! hello