input: Write a python function to calculate the average of three numbers.
  assert calculate_average(3, 6, 9) == 6.0
  assert calculate_average(10, 20, 30) == 20.0
  assert calculate_average(7, 14, 21) == 14.0
output: def calculate_average(a, b, c):
      return (a + b + c) / 3