Ans is 5/16.
The probability of getting 3 heads when the dice outcome is 5 is 1/6*10/32.
Probability of getting 3 heads when the dice outcome is x is: for x:3, P(3head|x=3)=1/8 , x:4, P(3head|x=4)=4/16, x:5,
P(3head|x=5)=10/32, x:6, P(3head|x=6)=20/64.
So the Probability is (1/6*10/32)/(1/8 + 4/16 + 10/32 + 20/64)=5/16.