I am due to go on holiday 3 weeks after the job is due to start, should I inform the interviewer, although I don’t want to put them off?

Yes. This happens more often than you think.
But consider what is most important to you, the job or the holiday. What will you do if offered the job and asked to cancel your holiday?
Most employers should honour pre booked holidays if disclosed in the interview process – but there are no set rules.