[Harlowe 1.2.3] Changing a Variable on a Link (Setter Links)?

Just starting out on Twine, so I'm not committed to Harlowe 1.2.3 (it just appears to be the default), so if this is easier to accomplish with another story format I have no qualms changing.

I'm trying to track a few variables across your choices, and once you reach the end, display a result based on your highest variable. From my reading this appears to be referred to as a setter link, but I can't seem to nail down the proper syntax.

Here's what a really basic example of what I would like to accomplish:

Choice 1 (variable A increases by 1) -> Next Passage
Choice 2 (variable B increases by 1) -> Next Passage
Choice 3 (variable C increases by 1) -> Next Passage

Next Passage
(A is highest) You prefer A
(B is highest) You prefer B
(C is highest) You prefer C

Hope that's clear.