Hey there! I'm in the exact same situation as you!
3.0 GPA isn't that bad, it won't get you into the more competitive schools. But it'll get you into a decent school, and your ACT test score isn't bad at all! (I basically have your grades but -2 points off the act... >_>;; )
Most people stress on unprepare-ness for college, but if available talk to your counselor in your school about your stress and worries. Or talk to students within your grade that are (if available) enrolled into a post secondary education option (PSEO) classes about how different college classes are different from high school courses.
I have a friend that's considering a gap year after they graduate because they feel exactly the way you feel. They've expressed to me that they feel overwhelmed about going into college right after graduating, and that a gap year will clear their mind. And maybe you're thinking that too! I'm not here to say you should do it or not. But you have to consider what you'll do during that GAP year that'll help you decide whether you want to continue towards college or not. And worst case scenario is that when you head back to college after that gap year you'll be rusty in all the subjects. But some people need that kind of rest.
What's nice is that some colleges let you submit essays you written for other classes as your college essay. Or you can go to community college for 2 years and transfer out to a 4 year uni. to finish up your credits. Saves money for you rather than going straight into a university. Plus if you apply for a large amount of schools at least
one of them will accept you. :>