Third point of support

My newest heatbed have holes in the middle at all 4 sides, so I can use 3 points of support for it - which should be a lot easier to level!
The middle support are created so it works without a need to be fastened.
2 two things at the other 2 support points are created to hold the nut, so its easier to level.
This things are created for the standard prusa bed holder with 3 slide bearings, even thou its shown with 4..
Regarding v2: When printed v1 was to small so this one is a little bigger - and it fits fine this time!