Did you have a charging system check? That should be done first. 3 hours is a short time and would only happen if you weren't fully charging the battery or had a huge drain somewhere. Check to see if your brake lights are not staying on, they draw a lot and a bad brake light switch would do it.
There is a test you can do at home, basically you make sure everything in the car is off. If you remove the negative battery terminal and strike it against the post and get a spark, (best done at night) you have a drain somewhere. Isolate it by taking out one fuse at a time and repeating the test until you get a no-spark condition. That will be the problem circuit.
Bob