Unless you have a 250A alternator you will not be running 3000W when the car is running, despite what your amp says.
You can not drain the battery whilst the car is running, as long as the alternator is charging properly. The battery will only drain when the amp is being used with the engine switched off (obviously). Provided it is wired correctly it will switch off with the headunit and wont cause any additional current draw or cause overnight flat battery etc.