Since no one has yet posted a scientific reason behind the 3rd gear mystery/misery, I want to theorize:
In first and second gears the engine vs. weight and gearing is up to great acceleration. In third, you start pushing major air. The combo of friction, weight, gearing, and torque is probably enough to tear up third gear, with the air resistance pushing it all over the edge. How's that for a theory? Now let's hope someone with facts chimes in!
David