For free?

Tomorrow I'm taking a few people with me to go to the new local bar where I know the owners. I have been informed that there is a schlager-party (!) and that every 3rd drink is for free. This poses a bit of a dilemma. Is it every third glass for free? What if I order a bottle of wine, will then every third bottle be for free? What if I only manage two bottles? Do I get no discount then? Oh, the problems of the modern world.