input: Write a python function to calculate the mean of three numbers.
  assert calculate_mean(1, 2, 3) == 2
  assert calculate_mean(10, 20, 30) == 20
  assert calculate_mean(-5, 5, 10) == 3.3333333333333335
output: def calculate_mean(a, b, c):
      return (a + b + c) / 3