input: Write a python function to calculate the product of three numbers.
  assert product_of_three_numbers(2, 3, 4) == 24
  assert product_of_three_numbers(1, 5, 6) == 30
  assert product_of_three_numbers(7, 8, 9) == 504
output: def product_of_three_numbers(a, b, c):
      return a * b * c