"wej paqwIj Datatlhpu'."
Translation:You have not returned my book yet.
Should "You have returned three of my books." work?
To me that's a little bit different because it could imply that you still have more of my books. But it could just mean, of the books I own, you have returned three (the ones that you had. Now if I can track down the other guy that has the rest...)
If it doesn't fit, how would you express my answer in Klingon?
Klingon doesn't do partitives in this way. wej paqwIj means only my three books, not three of my books. To express three of my books in Klingon, you have to establish that you have books, then you have to talk about three of them.
paq law' qanojpu', 'ach wej neH Datatlhpu'.
I lent you many books, but you have only returned three of them.
(Literally, I lent you many books, but you have returned only three.)
The of them translation is common when you use a number as a noun in this way, but remember that it's just a translation. The Klingon doesn't have it.
wa' yIHoH! Kill one of them!
mulegh cha'. Two of them see me.
In each of these cases, there was already talk about multiple people, so it's understood that when you say wa' or cha', you're talking about one or two of the people.