using System;

using System.Collections.Generic;       

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var list = new List<X>

        {

            new X {Code = "1234", ResCode = "ABC", TimingCode = "TAPOAC" },

            new X {Code = "1234", ResCode = "ABK", TimingCode = "TAQ123" },

            new X {Code = "1234", ResCode = "ABK", TimingCode = "TAQ124" },

            new X {Code = "1234", ResCode = "ABK", TimingCode = "TAQ125" },

            new X {Code = "1234", ResCode = "ABK", TimingCode = "TAQ126" },

        };

        Dictionary<string, Dictionary<string, List<string>>> result =

            list.GroupBy(x => x.ResCode)

                .ToDictionary(

                    x => x.Key,

                    x => x.GroupBy(y => y.TimingCode)

                          .ToDictionary(y => y.Key, y => y.Select(z => z.Code).ToList()));

    }

}

​

class X

{

    public string ResCode { get; set; }

    public string TimingCode { get; set; }

    public string Code { get; set; }

}

​