Mazda MX5 - 91 1.6 Battery charging/care? - No FM2R
Frankly for 3 months I'd just disconnect the battery. And then as and when I got around to it, simply reconnect and run the car a bit before disconnecting again.
I've left cars in airport car parks that long and they've never come to any noticeable harm - and nobody knows the battery (and therefore alarm) is disconnected.