Using ComboGroups for combination of 4 letter, group into 3 group with size (1, 1, 2).

The out put generate the results below.

comboGroups(letters[1:4],grpSizes = c(1, 1, 2))
Grp1 Grp2 Grp3 Grp3
[1,] "a" "b" "c" "d"
[2,] "a" "c" "b" "d"
[3,] "a" "d" "b" "c"

However, three more possible combination are missing. They are not the same combination with the above.
Grp1 Grp2 Grp3 Grp3
[4,] "b" "c" "a" "d"
[5,] "b" "d" "a" "b"
[6,] "c" "d" "a" "c"