I tried the other construction (number + o + plural noun), writing tri o ddolffiniaid , but the correct answer was tri dolffin.
Can the o + plural construction be used only with certain nouns? Or did I misspell some word?
The n + o + plural mthod is fine, especially with larger numbers, but it is becoming less used and less widely taught nowadays, at least on basic courses. For that reason, we have not routinely included it in this course.