I would think that if it were the clutch disc or flywheel, that it would occur in all gears, not just three. It also appears that the problem does occur on all three shift rails. Check the shifter base where it meets the shift rails. Couldn't tell you what to look for though.
The shift rails are free and move smooth. Sitting still it shifts fine, but when moving it has trouble like the syncros are sticking. Mike may well be rite, but I don't know how it worked for so long then all of a sudden went south. I am going to pull it apart tomorrow and look at it. I also need to see what is going on with 5th gear.
Thanks guys for your ideas, it helps some times to ask.