Does it make sense to use Aperture 3 in combination with iPhoto or can you drop iPhoto?
You should consider Aperture to be a replacement for iPhoto. When you switch to Aperture, you will import your iPhoto DB of photos into Aperture and work with your photos exclusively in Aperture. The features that you are used to in iPhoto will be available in Aperture.

Aperture still have a 30-day free trial available from Apple. 

What you can do is make a backup of your iphoto library (because backup are good, right), import the photos into Aperture and give it a try. Over the next 30 days, when you import new photos, import them into both Aperture and iPhoto. This way, if you don't like Aperture, you still have an up-to-date iPhoto library. If you are going to switch to Aperture, then you can delete your iPhoto library to get back that space.

I am a terrible photographer and I don't mind Aperture's way of organizing v. iPhoto. Trying to keep two applications in sync with photos, however, is not an ideal solution for long-term use, IMO.