Job interview on Wednesday?

I have a job interview on Wednesday for a summer job as a sales associate/cashier. I'm a college student who is taking up the job just to make some extra cash. One problem I'm running into, though, is that I have a prior commitment that I will need to take about 3 days off for in June. I'm unsure of when to bring this up. I was already asked about my availability over the phone today, so I don't think it's impossible that they'll ask for a starting date on Wednesday if I fit what they're looking for. Should I bring it up after discussion of a starting date? Should I wait until I get a job offer (if I do)? Thanks!

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