motion sensor lights constantly on, motion only, or off
The problem with a regular "3-way" switch it that it doesn't have an 'off' position where neither of the sides is connected. For that you'd need a 'center-off' switch like this Leviton 5685-2W.  

Then, as others have commented, it may not be a good idea to cut power to the sensor as it could take some time to get itself restarted and operational.  

So, how about this instead:





This way, the sensor is always powered, but you're switching the light between the sensor's output and the always-hot line (with the off position in the middle).
The down-side is that you'll need an extra wire in the cable so that you can bring the sensor output back to the switch.