Oh but the 3" disks have the window with the gate on the INSIDE of the disk... While it's much harder to break it than on the 3.5", once it does... Big sad.
I remember these disk from my Spectrum +3 . Indeed more hard and resistant that the 3.5" . Sad, that the format was on the losing side and never evolved beyond the 128k (or was 256k?) that could store on a single side.