Oh well, it's a damp Saturday and I have nothing else to do ...
You claimed that a glider only has 3 forces acting on it
I did, and it does.
yet in the very same article you linked it even states that the glider has 4 forces acting on it.
It doesn't. Read it again.
"The thrust is determined by the size and type of propulsion system used on the airplane and on the throttle setting selected by the pilot." (those emphases aren't mine, they appear in the original article).
The US convention is to use the term "airplane" to mean a powered, fixed-wing aircraft - I used to argue, once upon a time, that gliders were also "airplanes", but I was wrong about that, it's their word and they get to define it.
The terms "propulsion system" and "throttle" also give a pretty good clue as to what we're talking about here.
So please, one more time, tell us all what this mysterious 4th force (in addition to L/D/W) is that acts on a
glider in flight ?