GoneWest why don't you take thrust out of the equation and consider a glider. It has a downward force due to its Mass and Gravity.
Newtons third law of motion states that every force has an equal and opposite force known as the reaction.
So now we have a glider with weight acting vertically down and the reaction acting vertically upwards. 2 Forces.
For convenience, we resolve the reaction into 2 "man made" forces called lift and drag to explain how the glider flies.
If we add a third force Thrust, there will be an equal and opposite reaction to that force however, the two reactions do not coincide with the established definitions of Lift and Drag. If we combine our reactions into a total reaction, there are now 3 forces acting on our aeroplane, one of which can be resolved into the two forces we require for the purpose of explanation thus giving us the conventional 4 forces.
The reference to 3 forces is to make you think about the forces acting and realise that Lift and Drag are not equal and opposite reactions.