Here's why:
* It modifies a noun: It tells us how many of something there are. For example: "three cats," "three apples," "three days."
* It functions as a determiner: It specifies the quantity of the noun it modifies.
While "three" can also be used as a noun (e.g., "Three is my lucky number."), in the context of your question, it's acting as an adjective.