Ask your vet or other friends if they can recommend a pet-sitter; this way the cats could stay in your house and someone would come there and take care of them.
My cousin owns her own business doing this and my sister worked for another company doing this. You can pay them to just come once a day, every other day (however long you feel comfortable with leaving your cats alone) or you could even have them stay at your house (not sure if they'd do that for a whole three months, but my sister would do it for a weekend or so at a time, while also checking on the other animals assigned to her during the day).
Another option, and I can't remember the correct term, is called house swapping (?), I think- there's a website that sets it up, with people who are traveling and need a place to stay for an extended amount of time. It doesn't have to be a direct swap, so you wouldn't necessarily be staying in their place while they're at yours.
Ah, here's the site- Home Exchange!
There's a yearly membership fee and it says that usually it is a direct swap (they say it increases the level of trust between the swappers.) I don't know what your plans in Europe are, but maybe this is an option? There may also be similar services out there like this!