Carry a heavy lance
have whatever polearm you prefer to use when dehorsed on your back. this could be LHB, GLB, war spear, poleaxe, whatever.
Everyone here is really pessimistic, I've done several 3 riding builds and it is completely viable to be a dragoon with rouncey. Don't go riding around alone in the middle of nowhere like other cav, because that is where you'll encounter the situation everyone talking about *being outridden/dying*. It isn't a build designed for cav vs cav combat in open planes in the first place.
Stick closer to your team and it is terrifyingly effective. With proper horse control you'll never be outlanced anyways, since lancing is timing your thrust releases for when you and the enemy cav meet.
