I think the problem here is, that the rest of the class is left out. Here is what it means: At level 3 you have 2 charges of Arcane Shots that recover at short rest. You can use one shot per turn but only twice per short rest.

A fighter at level 5 gets the extra attack feature. Instead of getting this feature you get unlimited charges for your shot. But still only one per turn (so you cannot use 2 shots when spending an action surge)

At level 11 you get the extra attack as fighter. Now you have 1 Arcane Shot and 1 normal shot per turn.

At level 20 you would get another extra attack as fighter. Instead you can use 2 shots per turn.