input: Write a function to find the largest three numbers in a list. The function should return a list containing these three numbers in descending order. If the list has fewer than three numbers, return an empty list.
  assert largest_three_numbers([1, 6, 3, 8, 7, 4])==[8, 7, 6]
  assert largest_three_numbers([5, 2, 9, 1, 7])==[9, 7, 5]
  assert largest_three_numbers([10])==[]
  assert largest_three_numbers([])==[]
output: def largest_three_numbers(nums):
      if len(nums) < 3:
          return []
      return sorted(nums, reverse=True)[:3]