Thanks for your replies!
Sorry. I forgot to tell that I also will take 2 of my DSLRs with me, so the reason why I got myself a Gopro was just to make spontaneous filming, like Vlog, or in Airplanes easier, rather than having a big camera with a big lens in frond of me.
The other reason is just for some under-water-shots at the sea.
I recently spoke with a friend of mine, who also has a Gopro 3 (Black), and he said, 3 Batteries are enough for him. He recommended me to also buy a Triple-charger, that you dont have to wake up at night and change the batteries.
I think I´ll go like him.
But still thanks for your help!
Have a nice summer!