input: Write a python function to calculate the harmonic mean of three numbers.
  assert harmonic_mean(1, 2, 3) == 1.6363636363636365
  assert harmonic_mean(4, 4, 4) == 4.0
  assert harmonic_mean(10, 20, 30) == 15.0
output: def harmonic_mean(a, b, c):
      return 3 / (1/a + 1/b + 1/c)