Princess Diaries 3 is not available to watch anywhere, because the movie does not exist as of 2015. According to CNN, there have been reports that Disney will create a third film in the Princess Diaries series, but Disney has not officially confirmed nor denied these reports.
According to Entertainment Weekly, Princess Diaries 3 is not a movie that Disney is planning to create. The rumor that the movie would be released came from a twitter post by Heather Matarazzo. In the tweet, she suggested the third movie was being worked on. However, sources close to Disney clarified that the rumor was not true soon after Matarazzo's post.