This is actually covered in the tesla model 3 manual, on page 23 of the one that shows up when I google "tesla model 3 manual".
Specifically, the paragraph that addresses this says (bold emphasis mine)
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12V Power Socket Your Model 3 has a power socket located in the center console's rear compartment. The power socket is suitable for accessories requiring up to 12A continuous draw (16A peak).
NOTE: Power is available whenever the vehicle is considered "awake". The vehicle may be awake for many reasons. For example, when using features such as Summon, or when features such as Cabin Overheat Protection, Keep Climate On, Dog Mode, Sentry Mode, etc. are enabled.
The vehicle is also awake whenever the 12V battery is being charged or is in use, during HV charging, when the vehicle is communicating with the mobile app, etc. Leaving an accessory plugged in does not deplete the 12V battery.
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I know people dont like to "RTFM", but the tesla model 3 manual is pretty good, and covers a lot of ground. Since the model 3 is so different from other vehicles, I suggest everyone give it a read.
https://www.tesla.com/sites/default/files/model_3_owners_manual_north_america_en.pdf