"别人" and “他人” they are similar to each other. When they means "other person's", they have the same meaning while "别人" is more formal one, they all represent for the other people except you.
in "其他人",“其”repreaenting some person you mentioned before or you give some specialize terms to limited, so “其他人" means the person expect those you mentioned.
Here are some examples:
别人的东西
为他人着想
我了解我的朋友,不了解其他人