The three cases of personal pronouns are objective, possessive, and nominative.
I, we, you, he, she, it, they are nominative cases. They are used when a personal pronoun is used as the subject of a verb or as a predicate nominative.
Me, us, you, him, her, hers, its, their, and theirs are objective cases. They are used when the noun or pronoun is used as an direct or indirect object of a verb, or as the object of a preposition.
My, mine, our, ours, your, yours, his, her, hers, its, their, theirs are possessive cases. They are used to show ownership.