I’ve never heard “three fives are fifteen”; and not 100% on that but i think “thee fives MAKE fifteen” sounds better usually I’d say “Three times five is fifteen”/“three times five equals fifteen” so if you wanna check someone’s multiply table knowledge ask “what’s three times five?” or “what’s three multiplied by five?” if you wanna be extravagant 🧐

I would say it like this "What is three times five?"

thank you

No, because three fives can also be 555.