B.M.
Sounds like the 3 y/o has had a good model of manipulation and immaturity. Your fiance threatens to separate because he fears for his son's safety and he is blaming a 5 y/o and 6 y/o for his son's behavior. Threatening and blaming instead of seeing reality and looking for constructive ways to correct behavior would be warning signs for me that he may not be the best person for you or your children.