Theres a fewfixes in getting rid of that notchy 3rd gear. However this is more of a problem relating tontype s gearboxes where type r has the crunch of 2nd gear.

The simply cure but not prevention is to change mtf
More often or use a better product. I have been changing mtf fluid but theres a new usa company making a new short shifter with built in shift cable bushings and maybe cables also. Its on k20a.org