Don't know how they get three;
1. Lift 2. Weight (mass x gravity) 3. Thrust 4. Drag
Ummmm that's 4.
Um when dealing only with the wing some may argue that drag is only a function of lift, hence 3 forces, but when dealing with the whole airplane or a real environment there are 4 generalized forces and many more if you start dissecting them. Exempli gratia drag is the sum of profile + induced + parasite + form + etcetera......