I have elective succession in three kingdoms, it says all of them are going to go to my heir, but when I die my nephew took over and all the kingdoms split, what the hell?

The ai can and will change their vote all the time for elective succession. Unless you are winning by a lot, it is very possible for the voters to keep changing their mind. If you think you are on death's door it is good to use any hooks to make people change or confirm their vote. It only lasts 5 years though, which is why you can't rely on it the whole time.

If that is what happened, then if they picked different people for the different kingdoms, the realm will split because it is 3 separate kings now.