3C2 = 3
There are 3 possible combinations:
(2, 2, number other than 2)
(2, number other than 2, 2)
(number other than 2, 2, 2)
Hence you need the 3C2
Doing it the 2 unit way, the answer is:
(1/6.1/6.5/6) + (1/6.5/6.1/6) + (5/6.1/6.1/6)
But this just equals 3(1/6.1/6.5/6)...