input: Write a function to find the largest three numbers in a list. The function should return a list of these three largest numbers. If the list has less than three numbers, the function should return the largest numbers that are available. If the list is empty, the function should return an empty list.
  assert three_largest([1, 6, 3, 8, 7, 4])==[6, 7, 8]
  assert three_largest([9, 1, 5])==[5, 9]
  assert three_largest([10])==[10]
  assert three_largest([])==[]
  assert three_largest([4, 7, 2, 7])==[4, 7, 7]
output: def three_largest(nums):
      if not nums:
          return []
      largest = sorted(set(nums), reverse=True)[:3]
      return largest