To get the data into list

Question 1

Your code is only psudo code and will not work. However, I think this is what you are trying to do and provided a working example.

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);
    }
}

The output is: run example

------------------------------- EDIT FOR OP ---------------------------------- Note your code needs to have this:

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

as i've said in the comments

Question 2

Here's a solution based on Linq operators rather than explicit iteration. It uses the Batch operator from the MoreLinq project - it's available on NuGet.

The line to zip the L, I and B-value sequences together is a little ugly, but .NET doesn't have a built in 3-argument sequence zip (zip got added in .NET 4.0, an alternative implementation is in MoreLinq for earlier versions). I'd probably implement my own extension method.

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();

        }
    }
}