Newton's third law doesn't explain stalls – or, at least, not their suddenness.
As angle of attack increases (or speed decreases) there comes a certain point where the lift suddenly drops in a dramatic way that wouldn't make sense from a naive application of Newton's laws. What's really happened is that the airflow has separated from the wings and Bernoulli's principle no longer applies. That's when you get a stall, and the plane starts falling rather than flying.
As angle of attack increases (or speed decreases) there comes a certain point where the lift suddenly drops in a dramatic way that wouldn't make sense from a naive application of Newton's laws. What's really happened is that the airflow has separated from the wings and Bernoulli's principle no longer applies. That's when you get a stall, and the plane starts falling rather than flying.