1. Right-click on one of the planes, and while pressing down on your mouse (or trackpad), rotate the planes to see how the figure looks like from different angles by moving your mouse (or finger on your trackpad).
2. Let go of your cursor, and deselect the blue plane by clicking on the corresponding circle in the left menu. Notice how these two planes intersect. Right-click and rotate the figure again if needed.
3. Now click the circle in the left menu to make the blue plane reappear. Then deselect the green & red planes by clicking on the corresponding circles in the left menu. Now that the two planes are hidden, observe how the line of intersection between the green and red planes (the black line) intersects the blue plane.