"I bought three textbooks."
I tried 「教科書を三本買いました。」 and it wasn't accepted because I've used a different counter. Isn't 「本」 also a counter for books?
You're right, I stand corrected. 「本」 means "book" but is not a counter for books, and I got that mixed up.
After the particle. The particle has to come directly after the noun it describes, and the counter comes after the number. Think of the counter as meaning "of them", so 三冊 means "three of them". The correct order is: noun + particle + number + counter [教科書 + を + 三 + 冊]; the sentence literally being: Textbook (object) three of them (I) bought. It took me a while to get the hang of it.