I am using a 4mm thick mirror so, I designed this little clip to attach to the frame above the Z end stop. It allows you to get the bed the right height without moving the end stop, or cranking your bed way down. It should work just fine for 3mm mirrors too, as it creates a 3.5mm offset.

Optionally there is a hole for a M3 screw to make sure it doesn't move. Be careful with the screw as you could scratch the green.