My girlfriend and I have decided that every third weekend we will go to visit her parents. Can you tell me how to calculate the exact dates for these days?

You can do it yourself with a simple calendar, marking the date of each of your every third weekend, or if you don't want to spend time on it, you can use a special calculator that will do it for you. I myself use such tools quite often when I need to calculate something urgently. It will quickly help you calculate the dates you need, and you don't even have to put any effort into it https://www.calculator.io/date-calculator/.