using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<string> StoreData=new List<string>();

        StoreData.Add("10,1,100");

        StoreData.Add("9,2,200");

        StoreData.Add("10,2,700");

        StoreData.Add("8,3,600");

        StoreData.Add("8,2,100");

        StoreData.Add("8,1,500");

        var res = StoreData.GroupBy(s => s.Split(',')[0])

                           .Select(x => x.Key +"," + 

                                string.Join(",", Enumerable.Range(1,3)

                                    .Select(r => x.FirstOrDefault(z => z.Split(',')[1] == r + "") != null ? 

                                            x.FirstOrDefault(z => z.Split(',')[1] == r + "").Split(',')[2] : 

                                            "0")));

        foreach(var r in res)

            Console.WriteLine(r);

​

    }

}