It appear a problem using a Sony Alpha (700 850 900 etc) to tethered on Apple Aperture 3.
First of all be sure to have the Sony Remote Camera Control, it discontinued on Sony support site, find it on the original camera CD.
Set the camera in USB: PC REMOTE, connect the USB cable to the camera and test it.
To use directly Aperture 3, download this free plugin from Apple web site.
Run it, set the directory where to pick up the photos...and be sure to set the same in Sony Remote Camera Control Software, set an existing or new project for Aperture, where to import the photos, set if you want to import photos in Aperture library or only to refer it as master files and finish the installation.
Now your are ready. You can shot by the camera button or by the Remote Control button, after few second you will see the photo appear on Aperture selected project.
The plug in work with every DSLR having their property remote control software.
Hope it help