YOU SAID:

I think taking a 3 hour class was a mistake

INTO JAPANESE

3時間の授業を受けたのは間違いだったと思う

BACK INTO ENGLISH

I think it was a mistake to take a three-hour class.

INTO JAPANESE

3時間の授業を受けたのは間違いだったと思います。

BACK INTO ENGLISH

I think it was a mistake to take a three-hour class.