I'd strongly suspect your delivery valve is leaking in cyl #3. Those are reasonably easy to R&R, but I believe it entails removing the intake manifold on the OM606. It should be essentially the same procedure as the OM603, so search for threads that walk you through the process and the tools you'll need.

Also, you're probably due for an Italian tune-up, and best yet, there's no charge associated with that little procedure.