Здравей ,
Ето едно много базово решение, което дава 100т. в Judge, но се нуждае от много рефакториране  

using System;

namespace _03.Stream_Of_Letters 
{
    public class Stream_Of_Letters
    {
        static void Main(string[] args)
        {
            string command = Console.ReadLine();
            string result = string.Empty;
            string occurence = string.Empty;

            while (command != "End")
            {
                switch (command)
                {
                    case "c":
                        if (occurence.IndexOf("c") < 0)
                        {
                            occurence += command;
                            if (occurence.Length == 3)
                            {
                                result += " ";
                                occurence = string.Empty;
                            }
                        }
                        else
                        {
                            result += command;
                        }
                        break;
                    case "o":
                        if (occurence.IndexOf("o") < 0)
                        {
                            occurence += command;
                            if (occurence.Length == 3)
                            {
                                result += " ";
                                occurence = string.Empty;
                            }
                        }
                        else
                        {
                            result += command;
                        }
                        break;
                    case "n":
                        if (occurence.IndexOf("n") < 0)
                        {
                            occurence += command;
                            if (occurence.Length == 3)
                            {
                                result += " ";
                                occurence = string.Empty;
                            }
                        }
                        else
                        {
                            result += command;
                        }
                        break;
                    default:
                        if (command.Length == 1
                            && char.IsLetter(command[0])
                            && command[0] >= 65
                            && command[0] <= 122)
                        {
                            result += command[0];
                        }
                        break;
                }
                command = Console.ReadLine();
            }

            Console.WriteLine(result.Substring(0, result.LastIndexOf(" ") + 1));
        }
    }
}