Is it necessary to inform the husband that the reason for his anger and uttering the divorce (talaq) was the magic of separation (sihr al-tafreeq), especially after three divorces have occurred?

Magic is real and has an effect. It is a means by which the devils among humans and jinn cause corruption and separate loved ones. However, their plot is weak, and it can be treated by legitimate means. You bear no sin for not telling your husband that the cause of what is happening is the magic of separation, and you should encourage him to perform Ruqyah (Islamic incantation). It is not permissible to accuse anyone of performing magic without clear evidence, as ill-conjecture is forbidden. The divorce pronounced by someone whose mind is overcome by magic or anger does not count. However, if he pronounces it intentionally and consciously, his divorce counts, and it becomes an irrevocable divorce (baynunah kubra), and she is not lawful for him until she marries another husband.