Hi! About your question concerning the cross sections of 3D shapes, is there any question or exercise that you are having difficulty with? I can help you with understanding it or clearing any confusion regarding the topic. 

Yes, i want to know how to construct cross section of simple 3D figure (Cube,Tetrahedron, Pyramid, Prism) that goes through 3 points geometrically along with it's geometrical reasoning. And how to create it if any of the points are outside the 3D figure (the kind of case where you might need to project the point into the 3D figure faces). Making a video about it would be much much more helpful because im sure what i want to get explained can't be written shortly in an article (and it also need visualisation). I have been really struggling to find this topic in google and found no result at all

I will give some hard example where there is some points outside the cube just to show what i mean :

the points I,J,K doesnt have their coordinate given at all, to get other point it must be done via geometrical construction. These pictures are just example, not the question that i was asking for, what im asking is the general way to make cross section along how to tackle harder example like this