First work out the number of ways of selecting three candidates out of 15:
15×14×13/(1×2×3)=15C3=455.
There are 3 positions to fill and there are 6 ways to arrange the 3 chosen candidates for the shifts: 123, 132, 213, 231, 312, 321; so the number of different ways of filling the posts is 6×455=2730. This is also 15P3 the number of permutations of 3 candidates out of 15.