All possible combination of pairs + 1 triple group from an odd list?

Question

Starting with an odd list of students, let’s say 21 total:

cohort = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20] 

I