Boxsters do tend to run batteries down quickly if left for long without a battery tender, but a 3 month old battery should be able to be left for weeks, not days. Have you had a load test done? The battery could have a bad cell in which case you should be able to get a new one under warranty.
If the battery is still good (it probably is not), you need to make sure the alternator is charging it fully. Voltage should be around 14 or more, checked with a multimeter between the battery terminals with the engine running.
DO NOT disconnect either battery terminal with the engine running! Have you checked if there is a parasitic drain when everything is turned off? Check the amperage draw between the battery negative terminal and the negative wire. For this you do have to remove the wire from the terminal, but only with the engine
off. You should also remove the frunk light bulb so you don't get a reading from it. Here's a pretty good YouTube video:
https://www.youtube.com/watch?v=zdIKNnwEjIs
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