input: Write a function to find the largest three numbers in a given list. If the given list does not have at least three numbers, the function should return an empty list.
  assert largest_three_numbers([1, 6, 3, 8, 7, 4])==[6, 7, 8]
  assert largest_three_numbers([10, 20])==[]
  assert largest_three_numbers([3, 6, 1, 9, 7])==[6, 7, 9]
  assert largest_three_numbers([5])==[]
  assert largest_three_numbers([])==[]
output: def largest_three_numbers(nums):
      if len(nums) < 3:
          return []
      return sorted(nums, reverse=True)[:3]