No. You can't do that.
Aperture 3 (non trial) will immediately upgrade the library into it's own format. This will forever render it useless to Aperture 2. Since Apple doesn't want lawsuits from people who will then be forced to buy Aperture 3, you can't use an A2 library with the trial.
Right now, with Aperture 3 Trial, I'm importing my whole Aperture 2 library. It just makes a copy of it and imports it to Aperture 3 library format, leaving behind my Aperture 2 library intact.