internal class Program
{
    private static void Main(string[] args)
    {
        List<List<HMData>> data = new List<List<HMData>>();

        string[] Ls =
        {
           "L1","L2","L3","L4","L5","L6","L7","L8","L9","L10","L11","L12","L13","L14",
           "L15","L16","L17","L18","L19","L20","L21","L22","L23","L24","L25","L26","L27","L28"
        };
        string[] Is =
        {
           "I1","I2","I3","I4","I5","I6","I7","I8","I9","I10","I11","I12","I13","I14",
           "I15","I16","I17","I18","I19","I20","I21","I22","I23","I24","I25","I26","I27","I28"
        };
        string[] Bs =
        {
           "B1","B2","B3","B4","B5","B6","B7","B8","B9","B10","B11","B12","B13","B14",
           "B15","B16","B17","B18","B19","B20","B21","B22","B23","B24","B25","B26","B27","B28"
        };

        for (int k = 0; k < 4; k++)
        {
            List<HMData> Data_Content = new List<HMData>();
            for (int j = 0; j < 7; j++)
            {
                var l = Ls.ElementAt((k*7) + j);
                var i = Is.ElementAt((k*7) + j);
                var b = Bs.ElementAt((k*7) + j);

                Data_Content.Add(new HMData {x = l, y = i, z = b});
            }

            data.Add(Data_Content);
        }

        foreach (var item in data)
        {
            Console.Write("data=[");
            for (int i = 0; i < 6; i++)
            {
                Console.Write("{0},",item[i]);
            }
            Console.WriteLine("{0}]", item[6]);

        }
        Console.ReadLine();

    }
}


public class HMData
{
    public string x;
    public string y;
    public string z;

    public override string ToString()
    {
        return string.Format("{{{0},{1},{2}}}", x, y, z);
    }
}

for (int k = 0; k < 4; k++)
{
    List<HMData> Data_Content = new List<HMData>();
    ....
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using MoreLinq;

namespace ConsoleApplication1
{
    class Program
    {
        public class HMData     // Dummy HMDta class for test purposes
        {
            public string x { get; set; }
            public string y { get; set; }
            public string z { get; set; }
        }

        static void Main(string[] args)
        {
            var inputItems = new List<int>() { 1, 2, 3, 4 }; // Input items from XML document - dummy test list

            // L, I and B value sequences
            var LValues = new List<string>() {"L1","L2","L3","L4","L5","L6","L7","L8","L9","L10","L11","L12","L13","L14","L15","L16","L17","L18","L19","L20","L21","L22","L23","L24","L25","L26","L27","L28" };
            var IValues = new List<string>() {"I1","I2","I3","I4","I5","I6","I7","I8","I9","I10","I11","I12","I13","I14","I15","I16","I17","I18","I19","I20","I21","I22","I23","I24","I25","I26","I27","I28" };
            var BValues = new List<string>() {"B1","B2","B3","B4","B5","B6","B7","B8","B9","B10","B11","B12","B13","B14","B15","B16","B17","B18","B19","B20","B21","B22","B23","B24","B25","B26","B27","B28" };

            // Convert our 3 -value sequences into one sequence of HMData objects
            var zippedValues = LValues.Zip(IValues, (lvalue, bvalue) => Tuple.Create(lvalue, bvalue)).Zip(BValues, (pair, bvalue) => new HMData() { x = pair.Item1, y = pair.Item2, z = bvalue });

            // Split the sequence into length of length 7
            var batchedValues = zippedValues.Batch(7);

            // For each of the inputItems, output a batch from the -value sequences
            // (note we will stop output as soon as either the inputItems or the combined -value sequences are exhausted)
            var data = inputItems.Zip(batchedValues, (item, valueBatch) => valueBatch).ToArray();

        }
    }
}