thank you very much, but It's an exercise from lesson 3 (minna no nihongo), and the vocabulary at that point only includes "kyoshitsu" for classroom . The excercise is "でんわは どこですか。 And the answer is "in the classroom 3". But I'm not sure how is the correct way.
There are a number of ways to say this, but I don't think the exercise is asking you to take a stab at vocabulary you don't yet know. I think they are probably just expecting you to answer something like 辞書は301教室にあります。Sorry if that's a spoiler. If you haven't yet learned 301号 or 301号室, or 301番, etc..., then I'm sure the above answer is fine.