My opening tool broke from trying to open the back cover the second time. I followed the instructions and it shows pushing the spring and pulling up on the tool, second try the little nub broke. It’s not an issue if you have a thick safety pin to open the back cover as it’s more solid. The back cover is probably the worst thing about the MX3, other than that it’s an amazing phone.
I have a technique which i feel makes it easy to remove the cover:
1. Hold the phone in left hand so that most of the upper back part (camera, flash etc.) is covered by your palm and your left thumb rests about 2 cm above the speaker hole.
2. Apply firm but not too much pressure with the left thumb.
3. Use opener tool with right hand to push the spring button and pull back.
4. So now only the corner will open up making it easier.
P.S: Sorry for bad quality pic :P