A three wheeler is only more stable than two wheeler when both are stationary. But time keeps on moving hence both vehicles are always in motion and hence equally stable. Moreover a two wheeler is more flexible while taking a turn.
Also, all wheels in a three wheeler must of same size and quality for a stable drive. But in a three people marriage, that equality can't exist, not unless the women are brainwashed to such a extent that they are essentially robots and have no agency of their own. In a two people marriage also women are brainwashed to serve but at least she doesn't have an adversary in the relationship vying for the affection of the same man.
My analogy may have its limitations but the injustice done to the women in polygamous marriages is real. If you have to compete in a relationship then it isn't a relationship. A relationship should offer comfort not competition.