"Eu nu am cărțile despre care vorbesc ei."

I do not have the books of which they talk about -> should have been accepted

''I do not have the books they are talking about'' should be the best form

de ce nu merge dont

http://dictionary.cambridge.org/grammar/british-grammar/speak-or-talk so only talk should be accepted

Enuntul 1 si 3 sunt identice si sunt sinonime cu enuntul 2, in consecinta toate variantele ar trebui sa fie validate