using System;

using System.Linq;

​

class Data 

{

    public int UserId {get;set;}

    public int NbPoints {get;set;}

}

​

public class Program

{

    public static void Main()

    {

        var myData = new [] {

            new Data { UserId = 1, NbPoints = 10 },

            new Data { UserId = 2, NbPoints = 10 },

            new Data { UserId = 1, NbPoints = 5 },

            new Data { UserId = 3, NbPoints = 5 },

            new Data { UserId = 2, NbPoints = 40 },

            new Data { UserId = 1, NbPoints = 10 }

        };

        var groupByandSortedData = myData.GroupBy(r => r.UserId)

                            .Select(g => new { User = g.Key, ActualScore = g.Sum(s => s.NbPoints) })

                            .OrderByDescending(p => p.ActualScore);

        var ranking = 3;

        Console.WriteLine(groupByandSortedData.Skip(2).FirstOrDefault());

        Console.WriteLine(groupByandSortedData.ElementAtOrDefault(ranking-1));  // due to index starting from 0 

        Console.WriteLine(groupByandSortedData.ToList()[ranking-1]);

    }

}