Before Vatican II were there really three distinctions of sin: venial, grave, and mortal?

No. The Church has always taught that there is either venial sin or mortal sin. So if somebody comes along with another distinction, it is not Catholic teaching. Mortal means deadly; it is the state Adam was in after he sinned, because he became mortal, subject to death. When you commit a mortal sin it is a grave sin, the kind the Church considers capital, like murder, adultery, or homosexuality, and it kills your soul. The only way to get back the sanctifying grace you need to restore life to your soul is through the sacrament of confession, or baptism, which gives you spiritual life again and brings you back into the Church. Venial sins we commit every single day, but because we are saved by God's grace and have sanctifying grace, venial sin does not cause us to lose our salvation, because it does not relinquish the sanctifying grace in the soul. You can confess a venial sin the moment you commit it, directly to God, a hundred times a day if you have to, and you can also bring it to sacramental confession if you wish and receive the blessings that come with the sacrament. But you are not required to confess venial sin in the confessional, whereas you must go to the priest and receive absolution for any mortal sins you have committed. The difference is simply that venial is not serious and mortal is serious, and the Church defines that in the catechism and canon law.