This is very nearly a normal charicteristic of this transmission. Or perhaps better said, even when brand new or freshly rebuilt this trans has some tendency for a weak 3rd gear synchro. As others have posted if you do a search you will find a wealth of information on how to improve this.
Before you actually remove the trans for an overhaul though, try changing the trans fluid and see if it makes a difference. A fluid with an approprate level of friction modifier often makes this concern much more livable. Here is a receint post about the available fluids that can be used.
http://www.contour.org/ubbthreads/showflat.php?Cat=&Board=trouble&Number=662764&Forum=All_Forums&Words=150&Match=Username&Searchpage=0&Limit=25&Old=allposts&Main=661828&Search=true#Post662764 
