input: Write a python function to calculate the average of three numbers.
  assert calculate_average(55, 48, 65) == 56.0
  assert calculate_average(10, 20, 30) == 20.0
  assert calculate_average(100, 200, 300) == 200.0
output: def calculate_average(a, b, c):
      return (a + b + c) / 3