Here is how I made the 3rd plane.
first choose the 1st plane at the 35 degree angle, as the reference point for the plane insertion. Insert the new plane perpendicular to plane one, next click in the box for a second reference, use any point on the side where you want to insert the plane, and then click in the 3rd box reference point. Here you need to click on a point in the same plane as the second reference point. I've circled the two points I choose for the second plane in this snip. It's in .jpg format so hopefully you'll be able to see what I've done.