There are many threads here on SAF discussing this. The military is not surprised by experimental usage by people of your age. I advocate for honesty here, because this question gets asked on health histories and security questionnaires at regular intervals, and if you tell the truth, you don’t have to keep lies straight and worry about being found out.
That said, no one here can guarantee what DODMERB will determine. The key will be very limited use, and as long ago as possible, with no intent to continue using.
Unless your parents look over your shoulder, they will not know your responses. But - they are usually a good resource for your medical history for when you were younger. Remember not to over-answer questions; the criteria is if you were diagnosed with something by a medical provider. Go to the DODMERB site and read the items in the left-hand menu. Take your time; read carefully.
I do have to comment on how you described doing it the first time, and you “ended up” doing it twice more. That’s a passive description. Look in the mirror and say to yourself you chose to do it, it didn’t just happen and presumably no one forced you. Ask yourself why you repeated this behavior three times after the first, knowing you were in the process of applying to a military school. Do you have trouble resisting peer pressure? Do you seek relief from stress in this way? Do you take risks because it feels a little too good? These questions are just for your own internal discussion, not for responding here, and meant kindly, to help you navigate that challenging transition to adulthood and self-knowledge.
I applaud you for taking stock of your situation and planning to be honest. That is a great start.