input: Write a python function to determine the sum of all positive integers between 1 and N that are divisible by 3.
  assert sum_div_3(15) == 45
  assert sum_div_3(10) == 18
  assert sum_div_3(20) == 63
output: def sum_div_3(N):
      return sum(i for i in range(1, N+1) if i % 3 == 0)