input: Write a python function to find the third largest unique number in a list.
  assert third_largest([4, 1, 7, 3, 8, 4, 3, 9, 2]) == 7
  assert third_largest([10, 20, 20, 10, 30, 40, 50]) == 30
  assert third_largest([3, 3, 3, 3, 3]) == None
output: def third_largest(nums):
      unique_nums = list(set(nums))
      if len(unique_nums) < 3:
          return None
      unique_nums.sort(reverse=True)
      return unique_nums[2]