All are correct. To make sense of the third one, context would be needed. For example, imagine this conversation: A: I saw you driving a car. Whose car were you driving? B: Charlie. I was driving his.

"I was talking to her" is correct, and implies that you are talking to a female. "I was talking to him" is correct, and implies that you are talking to a male. "His" and "her" are object pronouns. "His" is a possessive pronoun. It indicates ownership. John has a book. He has a book. Whose book is it? It is his book. "His" is masculine. The feminine form is "her." Mary has a book. She has a book. Whose book it it? It is her book. You cannot say "I was talking to his" because "his" functions like an adjective, not like a noun.

All are correct with an exception of the last one. Which isn't really incorrect but rather incomplete.

I was talking to his wife/ daughter/ sister/ friend/ boss etc.

'Her' if you were talking to a girl or woman. 'Him' if you were talking to a boy or man.