If two planes intersect, their intersection is line. In your case the line of intersection is line QR (see attached diagram).
Three points are collinear when they lie on one line. Points Q and R lie on the line of intersection between planes P and QRS. Then point A must lie on the line QR and there are three possibilities for point A:
- A lies between points Q and R;
- A lies left to the both points;
- A lies right to the both points.
