I don't know much about passcodes on 3DS systems, but what happens if you get the passcode wrong? Does it alert your parents? Does it wipe the system? Does it lock you out after a certain number of attempts? Four digits isn't that long, so with persistence, you should be able to guess it, depending on how it is set up. I also find that my parents tend to use certain, meaningful numbers in their passwords, so if you know what sorts of codes your parents like to use for other things, that might help you here.
That being said, maybe you should have a discussion with your parents about why they felt the need to put parental controls on your 3DS. If you are indeed 18, you're legally an adult (at least most places; I don't know where you're from), and it seems strange to have your family restricting your video game habits when you're old enough to vote or join the military. Do you share the system with a younger sibling whom your parents want to "protect"? Or do your parents feel that you're spending too much time playing video games? Maybe you could work out a compromise.
Worst case, you could always get a job and buy yourself another 3DS.