I guess if you bought a link, you should check that it's up and active (works) Go to the site where you bought the link from, check the links to see if your site is there, click it. If its there and it works there is nothing more to do. You could also check the page rank of the site to make sure its PR3 by doing a search for a page rank checker Seems a bit strange to buy something if you don't know what its for.....
If you buy link from PR3 site is mean you have a backlink from this PR3 site which will provide you PR juice. However, if you are talking about PR3 domain, then you just develop it as normal