As we know that 3 dice are thrown. We want prob. of sum on three faces at least 5 i.e. some may be 5 or more. We will find prob. of sum on three faces not 5 or less, i.e. sum on faces is 3 and 4 (1,2 is not possible because of 3 dice).
No. of ways for sum on faces not 5 or more
=4[(1,1,1),(1,2,1),(1,1,2),(2,1,1)]
Total out comes
=216Prob. of not 5 or more
=Prob. of sum on three faces at least 5
1−==