I have a 98 Civic Ex 5 speed and sometimes the gears grind but its only when im going hard and shifting to third. What are all the possible things that could be wrong and what can i do to fix this cause it sucks *** having to slow down just to shift and then get on it again...

 

Synchros likely. You have to rebuild and replace the synchros.

You can try some GM Friction Modified Synchromesh, it won't cure it but it might mask it.

 

have you ever changed the fluid? tranny and clutch fluids?
might just be your clutch.

 

3rd gear synchro from bangen gears. the idea is to shift quick and smooth. not yanken and bangen like your trying to hit a cherry