umm.. the biggest thing is take it easy on it.. the clutch feels fine?? and its pushed into the floor everytime you shift right??? doesnt sound like a slave cylinder because you'd know if it was.. when mine went it leaked like a siv and i had to double clutch everytime i shifted up or down, sometimes had to clutch it like 6 times..
my guess is the syncro's are getting worn.. just keep fluid in it. thats probably all you can do without tearing it apart.. and if 3rd gear does go out then just skip it and go from 2nd to 4th to get you home.
i was driving a friends bush truck in the mountains back in january and was in 4low and went to shift into 3rd.. oh **** no 3rd gear haha so i shifted into 4th nope nothing there either... haha, the funny part was that there were no signs of grinding or anything.