There are actually very few ways you could get that total, so you could just work them all out. If (X, Y, Z) are the scores from the 1st, 2rd and 3rd dice respectively, then the possible outcomes that give a total of 16 are: (4, 6, 6), (6, 4, 6), (6, 6, 4), (5, 5, 6), (5, 6, 6) and (6, 5, 5). I can't think of any other combinations, but I'm open to correction if I've missed any. That's out of a total of 6 x 6 x 6 = 216 different possible outcomes - so you can now work out how many give the right answer as a fraction of the total number possible.