Hi
A car sitting for 2-3 weeks can drain a battery. The problem can lie in the battery, the alternator (thing that charges the battery when the engine runs) or in the car's electricals (parasitic drain). A modern car does draw on the battery in the normal course of operations - sometimes this draw is large due to an electrical fault (not fun to diagnose)
If this is an ongoing issue, it might make sense to get a multimeter so you can measure the amps / draw on the engine in different situations.
A quick google will give you some good generic videos on how to diagnose
Cheers