Why is this not tiúr fir when on another question it was triúr deartháireacha?
This is exactly what I came here to ask!
I wish I knew the answer, too. Is "fear" singular in form because it's being used adjectivally to describe the three people?
I suppose it's an un-lenited genitive plural form of "fear". Is that correct?
Yes. Look at the notes.
Triúr is a noun, so this actually means ‘three of men’, similar like in English pair of shoes, dozens of engineers…
Could this also be are working?
Yes, and it was accepted as well.
I once drove past a sign: Warning! Tree fellers at work.
I submitted "There are three men at work" but it was deemed incorrect. How then, would you say "There are three men at work" in Irish, in both the sense of working and/or the sense of being at their place of work ? GRMA
Your translation would also be correct; report it as an error when opportunity allows for you.
Wouldn't you normally say 'Tá triúr ag obair'?
you could , but triúr fear is being specific to men, while just triúr could be men and women