using System;

using System.Linq;

using System.Collections.ObjectModel;

using System.Collections.Specialized;

using System.Collections.Generic;

using System.Threading;

​

public class Program

{

    public static void Main()

    {

        string s = "abcdefghij";

        var split = s.Select((c, index) => new {c, index})

            .GroupBy(x => x.index/3)

            .Select(group => group.Select(elem => elem.c))

            .Select(chars => new string(chars.ToArray()));

        foreach (var str in split)

            Console.WriteLine(str);

    }

}