Here's the thing.. rape is without question one of the most horrible things you can do to someone. I don't deny that. But three years in jail is a long fucking time. People get slapped with twenty-five year sentences, life sentences, that's all you hear about on the news, so anything less begins to seem like "no big deal".
It is a big deal. It's a huge fucking deal. Prison is hell. And three years is a lot longer than the vast majority of prison sentences. For rape, I'm going to say that maybe three years is a bit light. But... step back and realize that just because you've been desensitized to think anything under fifteen years is "easy time", realize that three years of these peoples' lives are not only destroyed, and after they get out of prison, it's not any easier.
They have to tell the people whose neighborhoods they move to they're sex offenders, they're scorned by their family, friends, the public at large. It's hell for them to get a job. As sick and twisted as it is, they pay dearly, and only the sickest of fucks is going to go out and do it again after three to five years of hard time.