Sounds like a possible synchro problem. My third gear synchro is going bad I think, but I am still able to shift it, maybe your's is completely gone.. I noticed changing the tranny fluid helped this problem alot, but it's still noticable, I don't think that changing your tranny fluid is going to solve anything at this point, let us know if you get it figured out..
1990 Ford Probe LX 3.0L V6 :s6:
SEE IT HERE:
1989 Mazda RX-7 Turbo II 13B-T "FC3S"
Turbo RWD Rotary Powa!