Well if the PR of the blog is 3 then there are more chances of your topics being indexed in the main page of google so choose good topics or even good product to launch in the domain.
Initially try to attract as much as traffic as possible, with the PR it wouldn't be much of a problem and as soon as you are confident sell ad spaces that is a great earning. I am sure there will be advertisers who will be willing to buy the spaces with PR 3 and good traffic.