Learn Farsi: سه نفر در اتاق هستند.

نفر (nafar) is a counting word used specifically when you are counting people with numbers. مردم (mardom) means "people" in a collective sense (like a crowd or a nation's people) and is never used with numbers.

Since there are three people, why isn't a plural ending added to نفر (nafar)?

In Farsi, nouns that come immediately after a number always stay singular. You literally write "three person" (سه نفر) rather than using a plural form.

If نفر (nafar) is singular, why does the sentence end with the plural verb هستند (hastand)?

Even though the counting word stays singular, the subject "three people" is logically plural. When talking about humans, Farsi verbs must agree with this plural meaning, so we use the plural verb هستند (hastand).

Can I use the short ending اند (-and) instead of the full verb هستند (hastand)?

Yes! In spoken Farsi, you will often hear the shortened version attached to the previous word, making it در اتاقند (dar otaaq-and). Using the full word هستند (hastand) is just slightly more formal and emphasizes their presence in the room.