Program.cs

using System;
using System.Diagnostics;
using System.Threading;

class Program
{
    static void Main(string[] args)
    {
        var processes = Process.GetProcessesByName("Notepad3");

        var countdown = new CountdownEvent(processes.Length);

        foreach (var p in processes)
        {
            p.EnableRaisingEvents = true;
            p.Exited += delegate
            {
                countdown.Signal();
            };
        }

        Console.WriteLine($"Waiting for {processes.Length} processes to exit");
        countdown.Wait();

        Console.WriteLine("Exiting");
    }
}