Simple riddle?!

Question:

3 on three, 2 on two, 2 on one
what is it?

Additional Details

2 weeks ago
sorry i made a mistake
it should be
1 on one, 2on two, 3 on three, but only 2on four

Answers:

Consonants.

1 on one (the N)
2 on two (the T and W)
3 on three (the T, H and R)
2 on four (the F and R)

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