The three 3mm screws are Torx for me. Now I have to find the right driver for this.
I had to remove this screw in the previous step, otherwise I couldn't remove the back.
My device had a additional screw underneath the rubber piece. I had to remove the rubber piece partially and then I could remove the screw (silver one with a big, flat head). Now I could continue to remove the back. Also the 4-way scroll wheel and buttons are screwed to the back plate, so two screws this guide doesnt show.