Steve - A quick search of previous SM420 topics came across a couple of possibilities that may cause your popping out of 3rd gear.
This reply from 1 Ton tommy mentions that "Jumping out of 3rd gear is usually because of worn clutch-gear or rear mainshaft bearing or both".
There's another thread that includes a reply from
Busta1 that mentions "For jumping out of gear, check the detent ball spring." which are supposed to help hold the shift levers and gear in the proper positions.
Other should be along offer further advice soon.
