input: Write a python function to calculate the average of three given numbers.
  assert calculate_average(4, 9, 10) == 7.67
  assert calculate_average(1, 2, 3) == 2.0
  assert calculate_average(5, 10, 15) == 10.0
output: def calculate_average(a, b, c):
      return round((a + b + c) / 3, 2)