I think you left your biology notebook on the bleachers yesterday.
Your is a possessive pronoun. It is used before a noun to show ownership. You're is a contraction meaning you are. In option 1 and 2, if you're is replaced with you are the sentences do not make sense. In the last option, your is supposed to own the verb going. Verbs can't be owned, it needs to be you're (you are) going. Option 3 is the only correct use of the homophone your.