using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var values = new List<decimal>

        {

          1m,

          1.5m,

          2m,

          2.5m,

          3.0m,

          3.5m,

          4m,

          4.5m,

          5m

        };

        var result = values.GetPreviousN<decimal>(3);

        foreach(var item in result)

        {

            Console.WriteLine(string.Join(",",item));

        }

    }

}

​

public static class Extensions

{

    public static IEnumerable<IEnumerable<T>> GetPreviousN<T>(this IList<T> source,int count)

    {

        for(int i=0;i<source.Count;i++)

        {

            yield return source.Take(i).Reverse().Take(count);

        }

    }

}