Not quite,
Let the point where the plane is be P
F is the point on the ground directly below.
Obviously you must assume that the ground is flat so A,B,C, and F all are in the same plane
This also means that angles PFA, PFB, PFC are all 90 degrees.
This is becasue PF is vertical and AF, BF, and CF are all lines on the horizontal ground.
also,
r, s and t are ground distances to F (which is directly below the plane.)
Reason for edit.
I have to edit becasue of display errors, I think this happens when I use the less than sign to donote angles ...