The case in which one of the points is on an edge and the other two are inside faces is somewhat more difficult. Here we can't construct the trace of the section plane on any of the faces right away. The point L is on an edge and points K and M are inside faces.

1. Consider the auxiliary plane SKM cutting the edges AC and BC in points E and F respectively.
2. Draw KM, the trace of the section plane on the auxiliary plane, and mark point P, the intersection point of KM and EF.
3. Point P lies both in the section plane and in the plane ABC. But point L lies in these planes too. Therefore, PL is the trace of the section on the plane ABC, and so we can mark the intersection point of PL and BC.
4. Next we construct the trace of the section on the plane SBC and mark the point of its intersection with SC.
5. Finally, we draw the trace of the section on the plane SAC and mark the point of its intersection with SA.
6. Now we have all the four vertices of the section and can complete the construction.