They were three is a famous line from the French novel, The Three Musketeers. As the title indicates, it was about their number not their age.
So where do we report mistakes then.
There is a box next to the "comment" box, with a drop down menu, labeled "Report a problem".
Ils étaient trois. was accepted.
I think this is another example of a sentence that could be interpreted 2 ways and both answers should be accepted.