Main

 static void Main(string[] args)
  {
      
      long result=0;
      
      result+=3*multtiple(999, 3);              // it will first sum of multiples of 3 then multiply it by 3
      result+=5*multtiple(999, 5);              // same for 5
      result-=15*multtiple(999, 15);            // to remove repeting values(multiples of both 3 and 5)
      
      Console.WriteLine("Sum of multiples of 3 and 5 :" +result );
  }

Multiple

static long multiple(int range, int num)
{
    long n=0;
    for (int i = range; i > 0; i--)         //it will give us largest number within the range that is                                                                       
       if (i % num == 0)                    //either the multiple of 3 or 5 
         {
           n = i/num;
           break;
         }

   return (n*(n+1))/2;                      //It will five sumation of series till the range(explained below in series)
}