using System;

using System.Linq;

using System.Collections.ObjectModel;

using System.Collections.Specialized;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        ObservableCollection<string> collection = new ObservableCollection<string>();

        collection.CollectionChanged += (sender, args) => {

            if(args.Action == NotifyCollectionChangedAction.Add &&

                collection.Count == 3)

                HandleItemsAdded(args.NewItems.Cast<string>());

        };

        collection.Add("www.1.com");

        collection.Add("www.2.com");

        collection.Add("www.3.com");

    }

    public static void HandleItemsAdded(IEnumerable<string> newItems)

    {

        foreach(var item in newItems)

            Console.WriteLine(item);

    }

}