Learn Dutch: Wil jij appels? Ik heb er nog drie. - Do you want apples? I still have three of them.

Here, er replaces the noun that was just mentioned (appels). It translates to "of them" or "of it" in English. This is called the quantitative er.

Can I just say Ik heb nog drie like we say "I still have three" in English?

No, in Dutch, you cannot leave a number or quantity hanging without a noun. If you do not repeat the noun (appels), you are required to use er to replace it.

Why is the word order Ik heb er nog drie rather than putting er at the end?

The word er is a very "light" word that likes to sit as close to the working verb (heb) as possible. It generally goes right after the verb and before adverbs like nog (still) or niet (not).

What exactly does nog add to this sentence?

The word nog means "still". In this context, it gives the meaning of having items "left" or "remaining", as in "I still have three of them" or "I have three of them left."

Why is the plural of appel made with an -s (appels) instead of -en?