Without replacement
With combn, each vector appears in a column:
combn(LETTERS, 3)
# Showing only first 10.
     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,] "A"  "A"  "A"  "A"  "A"  "A"  "A"  "A"  &quo...