The sample space consists of these 10 possible rolls:

(1st roll) + (2nd roll) + (3rd roll) = 15

3+6+6 = 15
4+5+6 = 15
4+6+5 = 15
5+4+6 = 15
5+5+5 = 15
5+6+4 = 15
6+3+6 = 15
6+4+5 = 15
6+5+4 = 15
6+6+3 = 15

2 of those 10 have 4 on the first roll.

Answer: 2 out of 10 or  which reduces to .

Edwin