Always when I am designing 3d printed parts, I wonder what will be dimensions on the real part. Especially on holes for screw and nut slot.
I made a small analyse on the Prusa Mk3s STL files :

We can see that then is a range of size for the same purpose :
- M3 nut hole: 5.37 to 5.6mm
- M3 square but : 2mm thickness, width 5.6mm
- M3 hole : 3.2 to 3.8mm