Its not so much staying in after the 3 hours (if flight is delayed), its arriving in the lounge more than 3 hours before boarding time. Many locations have this rule and when I worked in Russia (Aeroflot lounges) it was exceedingly well enforced and no danger of getting lounge access more than 3 hours before your flight . My local airport in the UK is the same for (paying) guests.

Yesterday in Munich, notification of our flight change to the last flight of the day (8 1/2 delay) happened just as we were entering the lounge and the lady there made the comment, "this is ok, you can come in as you are platinum and can stay until your flight departs".