I would wait until the offer, otherwise why complicate things for no good reason.
However, you can ask in the interview what timeframe they have in mind for bringing someone on board, in terms of the length of the selection process and start.
You can usually negotiate a start date within 2-4 weeks from the time of offer.
Assuming the recruitment process lasts a couple months, this puts you pretty close to your 3+ months preferred timeframe.
If they ask you back when you are ready to start, don't tell about the 3 month constraint. The idea is not to make your timing constraint become a point of contention and provide reason for doubt on their end.
Rather, say that you have some ongoing projects you would need to wrap up but generally do not anticipate any delays or issues with starting within the expected timeframe. Don't give the impression that you are struggling with this point - just give a simple concise answer and move on. Good luck!