Hello, I hope I can offer you some assistance.
To start with check the battery voltage if as you suspect it is flat then before proceeding any further charge the battery. Once the battery is fully charged you can then investigate the cause of the flat battery. Start the car and check that the alternator is charging the battery, with the engine running there should be a reading of 13.5 to 14.5 volts a reading of less than 12.5 volts would indicate a charging concern.
Have the battery condition tested, most garages will do this free of charge and replace if necessary. A garage when replacing the battery should check the charge rate of the alternator after replacement as a routine.
If the battery and charging system are OK but the battery was flat then there is an electrical drain, have the current drain checked when the car is locked up idle, after around half an hour the drain on the battery should be no more than 0.05 amps but is usually around 0.02 amps, any higher would indicate a drain.
Even though the car is only 3 years old it is possible for a battery to fail within this period so the age can't really be used as a guide.
Let me know if I can assist further or if you have more questions regarding this and I'll reply as soon as I'm able to.