Finding unique letter combinations within a word

Question

I'm trying to find all the unique letter combinations, of 3 or more letters, in the word "several" using C#.

I only have the code to break it down into an array, but I can't figure out an algorithm to make combinations of it.

class Program
   {
      static void Main(string[] args)
       {
            string s = "several";
            char[] charArray = s.ToCharArray();
       }
    }