Nous ne prenons pas les fruits and nous ne prenons pas de fruits are both correct depending on the meaning of the sentence. Nous ne prenons pas des fruits is definitely always wrong.
Les fruits works if you are talking about specific fruits and if it were in the positive you would say "Nous prenons les fruits."
If, in the positive, you said "un/une/de/des/du/de la", then you would change it to "de" in the negative. Basically, if the fruits weren't specific fruits then, it would become "de."
So, if you sentence in the positive is "Nous prenons les fruits," it will just be "Nous ne prenons pas les fruits." But if it's "Nous prenons des fruits," it will become "Nous ne prenons pas de fruits.