With a 3 months notice period, should I wait until I get a job to resign or resign now?

Unless you are set on leaving your current position regardless of the interview outcome do not tender 1st. 3 months sound awfully long, are you in a special position? Have you checked ib with ex colleagues who has left the form on how they handled it?

Ask if the new company can wait or buy out your notice period. Provided that there is no non-compete clause in your contract.

Do not resign until you have a confirmed offer that you have accepted.