I think you will be fine for 3 months, ive left cars for that long.
ive also found it handy to get a slow trickle charger, one of those ones that wont actually charge a dead battery, but will just allow a small charge to trickle in and keep a battery topped up that's already healthy, it keeps it conditioned that way aswell.
I have a solar powered trickle charger, sits in the garage window and is connected to the race battery on the Trophy, makes sure its kept optimum whenever its required.
iirc its a make such as Caltek or the like, and normally pretty cheap to invest in.