using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

       public static IEnumerable<List<T>> GetPermutations<T>(List<T> items, int count)

   {

      for (var index = 0; index < items.Count; index++)

         if (count == 1)

            yield return new List<T> { items[index] };

         else

            foreach (var result in GetPermutations(items.Skip(index + 1).ToList(), count - 1))

               yield return new List<T> { items[index] }

                  .Concat(result)

                  .ToList();

   }

​

   public static IEnumerable<List<T>> GetAllValidCombinations<T>(List<T> items)

   {

      if (items.Count == 1)

          return new List<List<T>> {items};

​

      if (items.Count == 3)

         return GetPermutations(items, 2)

            .Concat(GetPermutations(items, 3))

            .ToList();

​

      return Enumerable.Empty<List<T>>();

   }

    public static void Main()

    {

         var results = GetAllValidCombinations(new List<int>() {1, 2, 3});

         foreach (var result in results)

            Console.WriteLine(string.Join(",",result));

    }

}