using System;

using System.Threading;

using System.Linq;

​

class Program

{

    static void Main()

    {

        object locker = new();

        int i = 0;

        bool finished = false;

        Thread[] threads = Enumerable.Range(0, 3).Select(remainder => new Thread(() =>

        {

            lock (locker)

            {

                try

                {

                    do

                    {

                        while (!finished && i % 3 != remainder) Monitor.Wait(locker);

                        if (finished) break;

                        Console.WriteLine($"Worker #{remainder} produced {i}");

                        Monitor.PulseAll(locker);

                    } while (++i < 20);

                }

                finally { finished = true; Monitor.PulseAll(locker); }

            }

        })).ToArray();

        Array.ForEach(threads, t => t.Start());

        Array.ForEach(threads, t => t.Join());

    }

}

​