Either the spring on the brake pedal is weak and not pulling the pedal back enough to contact the switch or more likely the switch is bad. It's easy to replace, it is at the top of the arm that goes down to the brake pedal. Two wires that plug in and it screws in with a locking nut to hold it. Should be $6-$8 somewhere around there.
if the battery is in good condition and the lights are standard then 3 hours might be enough so it will not be able to restart itself but this may vary according to battery size condition and lights wattage.
The head lights are the biggest long term drain on your battery. (The starter is a heavier drain, but you can't leave it on for very long.)
Either the switch was left in the wrong position, or the thing is defective. The dome lights draw enough current to drain the battery overnight.
you drain your battery when you leave your lights on
that keeps the dome lights on
Yes, if your car sits a few days at a time and your cars interior lights periodically come on it will drain your battery.
A short in the wiring.
Yes, a loose battery cable will prevent the alternator from charging the battery. The drain on the battery is not caused by the engine not running unless an accessory is left on. Most likely, the drain is caused by accessories (radio, lights, etc) while the engine is running due to the battery not receiving its charge from the alternator and then not having enough charge left to start the engine.
you leave the car radio wind dheild wipers and bright lights on.
No you sometimes need to charge your battery or get a boost if you battery runs low. Leaving your lights on for instance will totally drain the battery.
Battery drain would depend on what else is on, for example headlights, courtesy lights, glove box light etcetra. If there is nothing electrical on then there should be no drain.
A stuck relay in the car can drain the battery. A dead cell in the battery itself will drain the battery overnight. If this is the case the battery must be replaced as there is no way to fix it. To find out disconnect the negative battery cable for around 8 hours. If the battery goes dead it has a dead cell. If not then something is draining the battery. Anyplace that sells batteries can test the battery for you.