Hmm..so your saying you are having problems shifting into 3rd gear. Now I would obviously say this is while riding correct? DUHH, silly me..
but that sounds wierd. Also, do you shift with a clutch or no clutch? That is another factor. And yes IT IS okay to shift with no clutch, just only 1st to 2nd gear you have to use clutch, but try these:
1. While your riding, in 2nd gear, you wanna go into third gear, kinda pull the clutch halfway in, and then try shifting into 3rd.
2. Same thing as No. 1 but then dont pull in the clutch halfway at all, just try not using a clutch, and right when you accelerate, and you get off the throttle, shift up, like a normal (no clutch-use shifting)
3. Another thing try is when you are fully stopped, in your garage or anywhere, try holding the clutch 3/4 of the way, then from 1st gear, while standing still, go from neutral to 1st gear, then shift up to 2nd gear, and then try shifting into 3rd gear
If none of these work, cause I have encountered a problem similar to this, then you would have to take it in for servicing because that's not supposed to be happening!