This overlooks that A, B, and C are competitively selected, actively maneuvering to win, and influenced by previous results.
E.g., if B is slated to lose because C is attracting more liberal voters, B is likely to move left in order to capture those voters. Or if B2012 wasn't left enough B2016 may be someone more of that bent. Also, a disliked presidential candidate tends to depress the party's congressional holdings, which makes it more difficult to enact their agenda, and tends to setup a win for the opposing candidate in the next term.
Furthermore, in democracies votes don't merely decide outcomes, they legitimize them. If 100% of the population votes and 90% vote for Hitler because the alternative is Stalin, at that point he basically has a mandate. If only 5% of the population votes and 90% votes for Hitler, I would say an uprising is well within bounds.
Voting your conscience may not net you specific near term outcomes that you desire. But in the long run your vote is a commodity politicians value and they will shape their policies to obtain it. Unless, of course, they can obtain it some other way, such as simply by pointing out their opponent is on the left/right side. If you're going to lock in on that, then you've already spent your opinion as far as they're concerned. The only way to control politicians is to be completely willing to withhold the thing they need and value -- victory. If you let the parties blackmail *you* with outcomes, then they are the ones in control, and that will be obvious by the fact the people are only given choices (Hillary/Trump) that a sound majority of them dislike.