a noun is a word that names a person, place, thing, or idea. In this case, 'three' is a noun because it names the number. It’s a singular, countable noun that doesn’t take an article.
1. I have three cats.
2. There are three glass windows in my house.
3. I will need three hours of sleep tonight.
When speaking, it’s important to enunciate the 'r' sound in the word ('three,' and not 'thee') to ensure that your listener knows that you are indicating the number and not the article.