What's the most optimal way to see if a string is included in a list of strings?
If you really have just three words, then the performance most likely won't matter. In other words, unless profiling shows that this method is performance-critical, use the method that you find most readable.

Some other options you could consider:


Using a switch:

switch (word)
{
    case "Foo":
    case "Bar":
    case "FooBar":
        return true;
    default:
        return false;
}


This has the advantage that it can use compiler optimizations. (Under certain circumstances, the compiler will turn such switch into a Dictionary lookup.)
Use a HashSet&lt;string&gt; instead of an array. Looking up in a hash set is O(1), but O(n) for an array. This could make sense if you had large number of strings to search for.
Use a trie. This has the advantage of comparing only relevant parts of the input string. This could make sense if you had long input strings or if you huge number of strings to search for.