If you want the most asymptotically efficient way of finding whether a string is in a set of strings which can be pre-processed, you can consider using a trie. The construction time is linear (O(L), where L is the sum of all the strings lengths in three sets), and the lookup time is linear on the size of a string which you are looking for (which is, in your case, 14).
A binary search tree would also be a nice option, giving you a logarithmic (on the size of the string sets) performance. This can be a bit slower, but probably easier to implement. Note that pre-processing (inserting all the strings of three sets into a tree) takes N * log(N) time, where N is the sum of set sizes.
Do not use linear search in an array, it's too slow.