These three are confusing me a lot! When do we use which possessive?
The correct choice always depends on the object (that which is possessed), unlike in English, where it's the subject (that which possesses) which determines the choice. So "his head" and "her head" and "its head" are all "sa tête", because "tête" is feminine. Note an exception: even a female object takes "son" as the possessive pronoun, if it starts with a vowel (e.g. "son image").
Don't complain -- it's even more complicated in German, where both subject and object determine the possessive pronoun :)