To check if number is divisible by 3 or not you have to use the mode operator and if reminder is 0 it means it is divisible.
 
using System;
namespace ConsoleHub
{ 
    class Programs
    {        
        static void Main(string[] args)
            {  
                Console.WriteLine("Enter a Number:");
                int intDiv3 = Convert.ToInt16( Console.ReadLine());
                if (intDiv3 % 3 == 0)
                {
                    Console.WriteLine("The Number {0} is Divisible by 3", intDiv3);
                }
                else
                {
                    Console.WriteLine("The Number {0} is NOT Divisible by 3", intDiv3);
                }  
                Console.ReadLine();
            }        
    }    
}    
