The last Google Pagerank update is 4 Feb 2013, according to the source here: http://www.hobo-web.co.uk/google-pr-update/
So the page rank you see might not be the actual page rank right now. Now is already October. A lot can happen in 8 months. Maybe the domain you are looking at has a few high pr backlinks pointing to it in Feb. But now, those high pr backlinks are gone.
Of course, this is just my speculation. But the important point to take home is this: The PR you are seeing right now is from Feb. As long as Google don't update the pagerank, the tools you are using will not be able to tell you the actual page rank of a domain.
Most probably, the domain is no longer PR 3 but PR 0. Which is why it is so important to check the backlink profile of a domain before buying. Hopefully, you have not bought the domain yet.