Letters Combinations

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace LettersComb
{
    class LettersComb
    {
        static void Main(string[] args)
        {
            char firstletter = char.Parse(Console.ReadLine());
            char secondletter = char.Parse(Console.ReadLine());
            char thirdletter = char.Parse(Console.ReadLine());

            int counter = 0;
            string counter1 = string.Empty;

            for (char i = firstletter; i <= secondletter; i++)
            {
                for (char j = firstletter; j <= secondletter; j++)
                {
                    for (char k = firstletter; k <= secondletter; k++)
                    {
                        if (i != thirdletter && j != thirdletter && k != thirdletter)
                        {
                            counter1 = $"{i}{j}{k} ";
                            Console.Write(counter1);
                            counter++;
                        }
                    }
                }
            }
            Console.Write(counter);
            Console.WriteLine();
        }
    }
}