Or just drag the vertexes.
Indeed, I only needed to cut a few brushes, copied the rest and edited the vertices where necessary.
What's the point of 3 point clipping?
Some weird cuts can not be done in the X-, Y- or Z-plane. In these cases
you need to be able to specify the three points of your cutting plane.
You can use it to chop off the "nose" from a cube.