using System;

using System.Text;

using System.Text.RegularExpressions;                   

public class Program

{

    public static void Main()

    {

        string input = "AM0122200204";

        Console.WriteLine(Regex.Match(input,@"(.{3})\s*$"));

        // or

        Console.WriteLine(input.Substring(input.Length - 3));

    }

}