Well, if it shifts properly from gear 2 to 1 but not from 3 to 2 than there is something wrong with the gear 2 synchro ring. How old is that transmission? I think that worn synchro rings of gear 2 is a very common case.

If linkage is odd, you would have problems to change from gear 3 to 2 before it's grinding. There is a lock to prevent engaging gear 2 as long as gear 3 (or 4) is in.

How about "slipping out" gear 2 when car is decelerating (hill down and throttle closed)?

I strongly recommend to consult the guys from brewers performance - they seem to be the gurus for me...