Hi, my name is*****'m an attorney and a former prosecutor. I am so sorry to hear of your experience. Unfortunately, you're not alone. Here is what I can tell you in response to your questions.
1. Your biggest challenge is going to be proving that the incident occurred in the first place. You've never told anyone, and I'm guessing the only witnesses were the three classmates who assaulted you. If they are still around, and if you are still in touch with them, you might try to get them to make admissions to you. This would have to be supervised by a law enforcement agency and make use of recorded phone calls. Also, any friends or family members who could recall changes in your behavior, mood, etc., when the incident occurred might help in establishing circumstantial evidence that something happened to you. And if you began receiving any kind of counseling or treatment, that would be another level of proof.
2. Remember that the people who did this to you were juveniles. Even if they could have been prosecute, it would have been under the juvenile laws of your state.
3. Generally speaking, an assault of this nature would be governed by the statute of limitations for a felony assault. In most jurisdictions that would be three years. However, if the evidence supported a charge of attempted murder, there would be no statute of limitations and a prosecution could be initiated at any time. But remember: it would be a juvenile prosecution. There is no jurisdiction that would treat people of this age as adults no matter what the offense. You may have heard about extending the statute of limitations in certain cases, but those are usually in cases of sexual assault. In your case, you were a victim of bullying.
4. Your state should have crime victim support services. Also, the National Center for Victims of Crime could direct you to support services. You can visit them here: https://victimsofcrime.org.
In summary: as a prosecutor, I do not see a viable prosecution for attempted murder. Therefore, I do not see a way around the statute of limitations problem. That doesn't mean that there aren't support services available for you. Likewise, the statute of limitations would bar a civil action as well. But don't give up. Start by going to your local police agency and find out if there are referral services available.
I hope this has been helpful for you.