The trio of acts join One Direction, Rihanna, Emeli Sande and Kylie Minogue.
Clarkson will be performing on the Saturday with Minogue and Ora.
Sande, Rihanna and One Direction will play on the Sunday.
None of the special guests are duetting with the finalists this year.
Instead, James Arthur and Jahmene Douglas will perform with their mentor Nicole Scherzinger, while Christopher Maloney will partner with his mentor Gary Barlow.
This year's X Factor final is taking place at Manchester's Central Arena in front of an audience of 10,000 fans.
X Factor finalists switch on Downing Street lights - Photo Gallery: