input: Write a python function to generate a random number that is divisible by 3.
  assert generate_random_divisible_number() % 3 == 0
  assert isinstance(generate_random_divisible_number(), int)
output: import random
  
  def generate_random_divisible_number():
      while True:
          num = random.randint(1, 1000)
          if num % 3 == 0:
              return num