There is absolutely no problem with her being 15 and you 18. In fact, almost all successful relationships I've seen have had that similar age gap.
Just because your dad might be 45 and your mom is 42, doesn't mean that's chill at our age.
What if a high schooler was dating a middle schooler? Not weird at all I suppose. I mean, if the OP is in love; love is love. So love. Otherwise, an age gap of 3 years... Ehhhh... My parents are 10 years apart, and they hardly get along.
OP, if you and your lady friend are like the Romeo and Juliet of our time, then go for it. Cops won't bust down your door if you two have love privately, just don't rape her or piss her off... Then she can press charges, provided she has proof.
critical edit: if your not sure if she feels the same way about you...But if your not sure, either bring it up, or tease her: play footsies,
goofily say "I love you" (her response to something like that is critical. Facial expression is also a key part)
Pick a flower for her. If she's a friend, she'll say "awh thanks OP your so sweet", or potentially "its so pretty oh my gosh thank you your so great blabbity blabbity bla." and then hug you.
Best of luck
tiger woods OP.