The cable to the right that moves well is your Shift Cable for forward/backward motion of the shifter. Gates 1-2, 3-4, 5-R. The one that doesn't move very much, to the left, is the Selector Cable, operated by side to side motion of the shifter lever. It positions the levers on the transmission for one of the 3 gates.
1 3 5
|-|-| <These are the Gates.
2 4 R
Since your symptoms are on the same gate, it may be that the synchronizer for 3rd and 4th is beginning to fail, but don't assume that until the cables have been adjusted and the problem continues.
[This message has been edited by fierofool (edited 08-15-2017).]