Question

Does Trick or Treat make you 3 types?

Answer 1

If you use it on a single type, it becomes dual type. If used on a double type, it becomes a triple type Pokemon. All this move does is add the ghost type to the Pokemon you use this move on.

Comments

so does that mean you can somehow become more than 5 times weak?

---

Yes its possible unless you use it on a pokemon that's already a ghost type.

---

So, what if a pokemon uses Trick or Treat on a protean Greninja and Greninja uses say a water type move? Will Greninja be a Water/Ghost type or a pure water type?

---

that is a question on it's own, you should ask it!