What's your experience driving standard transmissions and do you really know how to "double clutch"?
Forums (and especially the University of YouTube) cannot teach you every thing.
What happens when you try to put it into third gear? Is it just stiff? Does it grind? Does it happen every time you go into third gear, or primarily on an up-shift (2 > 3) or on a down-shift (4>3)? If the engine is off and the vehicle stationary will it go into third with out difficulty?
The problem is unlikely to be clutch related if it's only happening in third gear - clutch related shift issues will typically show up with first & reverse. If it's third gear alone, and more common on a down-shift it's probably worn synchro rings and double clutching is a viable solution, provided you know how to do it correctly.
Fourth gear > depress the clutch > shift to neutral > release the clutch > tap the throttle to bring the engine rpms up > depress the clutch > shift to third > release the clutch - experience will teach you how much of a tap on the throttle you'll need, it varies with how fast you're going at the time of the down shift, and be aware, the up-shifting sequence is slightly different.