I've been a software contractor for years and often end up in a similar situation - interviewing for several roles and you're waiting on your preferred choice before declining another offer.
When I was younger I would have told a white lie in this situation because I would have felt awkward saying to a recruitment person that I had another preference or turning down an offer after all the work to get to that stage.
But I've learnt that it works out better if you're honest. Even if you end up taking the 3rd ranked job they will appreciate your honesty with them. Also it means should something happen (like you need another round with job preference 1 or it's postponed) you can go back and tell them honestly why it will be even longer before you make a decision. You will not the be the first or the last person who will have turned down a job at the final offer stage. It happens more than you think.
You're not doing anything wrong. You're not gaming anyone to get a better rate. Recruitment is a process where they know they're competing with other employers to get that employee as much as potential employees are competing for the job.