input: Write a python function to calculate the grade based on three subject scores.
  assert calculate_grade(85, 70, 90) == 'A'
  assert calculate_grade(60, 75, 80) == 'B'
  assert calculate_grade(55, 60, 65) == 'C'
  assert calculate_grade(45, 50, 55) == 'D'
  assert calculate_grade(30, 40, 50) == 'F'
output: def calculate_grade(score1, score2, score3):
      average = (score1 + score2 + score3) / 3
      if average >= 80:
          return 'A'
      elif average >= 70:
          return 'B'
      elif average >= 60:
          return 'C'
      elif average >= 50:
          return 'D'
      else:
          return 'F'