I'm 20 and falling for someone over 3 years younger than me
Why are you falling for someone who is only 17?
She's only 17, but I love her.
It's not morally wrong to fall in love with someone who is younger than you, but it's not legal to have sexual relations with someone who is under 18 in most states.  You should wait until she turns 18 before having relations with her because it could get you in a lot of trouble, and people might think it's strange.
We can't just make out. We're trying to feel each other out. It's fine if she's 17, if we just don't tell anyone.
Having sex with a girl that is underage is still against the law, and hiding it will only prove that you knew it was wrong. If you have sex with her you could go to jail.