This girl that I don't like is dating my crush. Should I tell the guy that she's pregnant and cheating on him to break them up, even though she is none of those things?

Absolutely not. Both of those things can be disproved, and will leave your crush unable to trust you when he inevitably discovers that you lied to him. Doing so makes you look jealous and manipulative (which are not traits that people want in their romantic partners), and could actually result in your crush and his girlfriend becoming closer to each other as they work through the conflict. A plan like this is highly likely to backfire on you, and won't have positive results. Take a break from these people instead, and talk to someone you trust about how you feel.