It would seem that a couple DJI Phantom 3 pilots have this issue right out the box, which is possibly caused by either the inital IMU calibration not being perfect at the factory, or the fact that that the box suffered a few bumps during shipping. Either way its fairly easy to fix via a IMU calibration.
Connect to your Phantom 3 drone via the DJI pilot app, and open the settings menu (upper right side of the app screen). Then click the icon, and scroll to the sensors section and click on the IMU Advanced button
Before comleting the calibration make sure your Phantom 3 quadcopter is located on a perfectly level and stable surface.
Once the calibration is compete you should see the IMU calibration OK writing and then you will find that the camera on your phantom drone is now perfectly level again so you can get back to taking awesome aerial photographs!