try this
Why is the car alarm going off almost every night around 3:00 a.m ?
Either she comes or go to work at that time, or the owner of one of the other cars does, and do something (walking to close to her car or slamming his door) that triggers the hyper-sensitive alarm.
You should watch at 3 am why it goes on. You probably did otherwise how do you know what car is going off ? but you watched after it went on so you don't know what triggers it; you should watch before it goes on.
From my own experience, (and I'm fighting noise for over 20 years) it never or almost never helps to talk to the polluter.
You can call the police. It is a nuisance and this is a misdemeanor.
If the alarm doesn't stop after 1 minute, that's another infraction.
You might think the police won't get involved in this; you might be surprised. they might drive by around 2.45 a.m. and sit in the car for 30 minutes to see what's going on. Depends on the officer (he may hate noise as well).
It is better to go to the station and talk to someone rather than using the phone.
You can file a complain (they must take it, they can't refuse to take a complain)
Then they will have to talk to her and that might be enough. She can receive a ticket.
Assuming you both rent the apartment, the lease mention that the tenant will not be a nuisance to others. The owner can do something about it if he/she wants. Like forcing her to disconnect the alarm (they are useless anyway; almost everyone has an alarm and yet a car is stolen every 7 seconds !)
Or park elsewhere (in the street for example).
And you are entitled to peace and quiet; read your lease, her is the same as yours. First check if your building is rent control. If it's not, the owner can evict you without giving a reason (if you annoy him and he prefers to evict you rather than confronting the polluter). if it's rent control, the tenants has a lot of power (it is almost impossible to evict you) so go ahead and pressure him, gently and politely at first. less gently later.
The owner promised you in the lease peace and quiet . You can pay less rent deduct 100-200 $ for example and tell him why until he fixes the problem since there is a lower quality of life by him not resolving the problem. It is legal to do so but you will really piss him off.
It all depends how far you want to go.
Try the police first.
The above advise is for you, all legal.
I, personally, would not have this kind of problem; if the alarm goes on more than 2 nights, I will not complain, I will not talk to anyone but there will be no car present the third night (don't ask...)