3 months eh?
You'll really want to have the car plugged in, i.e. be charging the car. Not just the 12V battery.
Depending on your settings, even a fully charged battery (which isn't recommended for storage anyways, more like 60%) may not last unplugged for 3 months. You'd have to disable darn near everything you can, never "check on it" via the app, and make sure it's stored somewhere where the temps are somewhat controlled (32F - 100F or so).
Then it
might be OK upon return.
But if you can plug in a battery tender, hopefully you can also plug in the car (maybe it's not feasible due to a shared circuit or something?).
Standby drain according to the Stats app,
when plugged in, works out to about 60W average. This will be mostly the 12V system, and implies you'd need a 5A charger. However, leaving it unplugged and turning off a bunch of features, this drops to about 11-14W. The 1.5A charger would be sufficient in this case.
I'd be curious if just keeping the 12V battery topped would prevent drain on the HV battery that has been otherwise noted (it should), but you probably don't want to be the guinea pig on that. But if Tesla service recommended it to you, then... hey, why not? (peace of mind, that's why)
the thing that confuses me about this recommendation from tesla, is that if you could plug in a battery trickle charger somewhere, you could plug in the car charger instead into the same outlet. Since the HV battery charges the 12v when plugged in, I cant see any benefit for using a trickle charger on a tesla (Because of the above).
Am I wrong in my understanding that the HV charges the 12v? If not, am I wrong in assuming that if one could plug in a trickle charger to an outlet? If neither of those are wrong, what would be the point of a trickle charger?
(real question)
I know what they are for, as I have one for my BMW, but I cant think of why one would be needed on Model 3.
I expected this response from someone
much sooner, heh. It's possible they're storing on a shared circuit that plugging in the charger would trip a breaker or something.
I can see benefits, but they're small and I'd still be concerned about the HV battery state over time. It's a risky bet, if probably a safe one. Keeping the 12V topped up should prevent drain from the HV battery, while keeping the 12V battery happier overall.
The "should prevent drain from the HV battery" comes with so many caveats though. They're all possible to deal with, but I'd fear not accounting for something.