input: Write a python function to calculate the mean of three numbers.
  assert get_mean(1, 2, 3) == 2.0
  assert get_mean(4.5, 3.5, 2.0) == 3.3333333333333335
  assert get_mean(-1, 0, 1) == 0.0
output: def get_mean(a, b, c):
      return (a + b + c) / 3