How do u clip things with 3 Points, for example, cut a concave triangle wedge out of a brush 16 wide ( 4 faces, making it 5) so then u could cut a convex triangle from another brush and slot them together.
The third point is used by rotating your 2d view and then placing the third dot. Not, that I ever use it. It probably has something to do with not being able to cut at certian angles without rotating the brush (using only two dots).
Yes with 2 points you can only clip on planes parellel to the x, y or z axis, by using the third dot you can clip along any arbitrary plane it takes a bit of effort to understand what you're doing though...
Place the 3 dots and then switch to another axis and manipulate the dots and you will see how it works ( still not easy to use, but I've actually had great use of it sometimes )