If you want to improve PR rank 3 without backlinks, it's possible. But your site have high quality unique content without over optimization means no keyword stuffing, Keyword stuffing means obtain maximum search engine ranking and visibility for particular phrases or word in your content so be careful write content because you doesn't create any backlinks, But I will suggest to you create backlinks because Google page rank is mainly based on back links or fast improve PR ranks, if you use backlinks that's good for you otherwise hire content writer then start write high quality unique content. If you plan to create backlinks so check this point create profile in forums, forums are a great way to achieve links to your website, directory submission , Links from related websites, Blog commenting, Social Bookmarking, Article Submission.