The problem here is battle set up. In real life you place your cavalry in the position you NEED them to be. They don't so much target a "unit" as they target part to the line that you want smashed, opening up a gap to be exploited. BM battles simply don't replicate this so well, a 3 man unit is actually kind of ideal for cavalry to targets, since it would be easy to crush that part of the line, opening a gap for infantry to exploit while the cavalry can ride through and regroup for another charge. Since BM doesn't provide a way to select the position in the line of your troops, luck will be the factor.
Remember in RL you don't position Cavalry to produce the "most kills from hits scored" that is a game mechanic. You place them to maximise the strategic gain of the charge. Depending on the situation this might be targeting the strong middle of the line, it might be exploiting a weak flank, most often it was trying to target the part of the line you are trying to weaken, that way when your archers and infantry has whittled away the troops their a heavy cavalry charge can quickly route them.
So since BM doesn't replicate real battle that well charging a 3 man unit is pointless where as real life may be good. To compensate for that, cavalry charge where they need to be should be shown by given priority to make sure they do a real charge when possible.
