Just tell them you'll be gone within the hour and they won't care. The "3 hour" or similar rules are far more about the length of time you're in the lounge rather than when your flight actually departs - or at least that's been my experience.

As a not-particularly-useful datapoint, I was rejected access to one of the T3 lounges on arrival a few months. However I completely forget which lounge it was, so as I said it's not very useful...