Taken from Tennant's fan page blog:
An image from the preview clip from the Doctor Who 50th Anniversary Special that was shown yesterday at the Guardian Edinburgh International Television Festival has appeared on the internet.
Although the quality is quite low,the picture clearly shows three incarnations of the Doctor played by Matt Smith, David Tennant and John Hurt in a scene together.
In addition, Boyd Hilton, Heat Magazine's TV and reviews editor tweeted a fragment of dialogue:
John Hurt's Doctor to Matt Smith & David Tennant's Doctors: "I'm looking for the Doctor"
Tennant: "Well you've come to the right place"