"Three is a number."
Translation:Drei ist eine Nummer.
I found this:
A Nummer is a numeric identifier or a rank, a Zahl is a quantity.
Nummern designate something specific, such as a phone number, or indicate the order of something, such as house numbers or room numbers. In contrast to Zahlen, you cannot apply arithmetic operators to Nummern. You may, for instance, add two telephone numbers (Telefonnummern) as if they were Zahlen, but the result is completely meaningless.
In this sentence both "Zahl" and "Nummer" should be accepted. The use of Nummer is very common in many situations where it is the only correct word to use: phone number, house number, credit card number, order number, number in a race, article number, etc. Right now, I can't think of any good examples for Zahl other than 'page number' and the use in mathematics...