input: Write a python function to calculate the mean of three numbers.
  assert mean_of_three(2, 4, 6) == 4.0
  assert mean_of_three(1, 1, 1) == 1.0
  assert mean_of_three(5, 10, 15) == 10.0
output: def mean_of_three(a, b, c):
      return (a + b + c) / 3