For finding the number of targeting say x unique ships, you can use the the combinatorial identity 3CN which lists the number of ways of picking N things from 3 (so 3C2 will be the number of ways of picking 2 ships from 3 original ones which is 3C2 = 6).
This formula can be used to get the number of combinations of ships from a total number and you can extend this beyond three.
Also I have to ask: is this probability going to be skewed by the geometric distribution of ships? (I.e. will be based on radius and location information instead of the above assumptions)?