I have a small presentation on epistrophe coming up and I want to make sure I don't say anything incorrect. Would the sentence "Ever since I left the city you, you, you" (it's from the song "Hotline Bling" by Drake) count as epistrophe? The one part I'm not sure about is the fact that "you, you, you" isn't three different clauses because they don't each have a subject and a verb.
No, that is not an example of Epistrophe.
Epistrophe is rhetorical repetition that occurs across different sentences, phrases, or clauses. Your example is repetition for meter and emphasis, not for rhetoric. It doesn't occur in different sentences or clauses, but repeats the end of one sentence.
You are correct to observe that "you, you, you" does not represent three different clauses, because there are not three verbs.