<< do you mean purchase them from a bookstore and give them to the library? or buy them from the library? >>
I think he means you just pay for the price of the books.
Because if you could buy the books at a bookstore and give them to the library, why not just look for the lost 1 and return all 3...or buy a copy of the lost 1 and return it with the other 2.
The library has probably replaced them by now and just want as much $$ as they can get from you.