I don't know much about SDA, but I don't think Mormon or JW are cults. I don't agree with them, but I have to hold out hope that God saves us even if we don't understand everything right. That by faith in Jesus, we are given Salvation. Because if that's not true, then there's no hope for any of us. Mormon's have extra books along with the bible. Those books I think are a sham, but I don't think they refuse their faith in Jesus or in God. Jehovah Witnesses studied the bible and developed many other theologies that aren't shared among other Christian groups, but I think their faith and trust in Jesus, and in God is still very real.
I think you need to sit down and discuss Jesus with both these groups and then see if you come away with the same beliefs you have now.