I recently found out that I have adhd (inattentive type) that has tremendously impacted me mentally. It certainly showed in quizzes/tests/exams and in my grades. I am still a junior and I haven’t taken the ACT’s and SAT’s luckily. My psychiatrist believes I will do a LOT better in school (considering I am already top 10% of my class) after taking (prescribed) medication. Because of how big of a role my ADHD has played in high school, should I let colleges know when applying? I am afraid that they might “view me differently” but at the same time, they should know why I have 2 or 3 B’s.
2 or 3 Bs is hardly any Bs at all. It’s probably not worth making excuses.
I understand that it doesn’t seem like a big deal, but I plan on applying to competitive colleges where the acceptance rates are usually below (or around) 30%. It isn’t considered an excuse if I’ve been diagnosed. 
2 or 3 Bs isn’t going to change their perception of you. If you decide to talk about it, you could certainly add a line or two in the ‘additional information’ section of the application, but you want your application to be a case for why you would be an asset to their school - what you have to offer them, the kind of person you are, your passions and interests. Talking about a ‘challenge overcome’ is generally only recommended if that challenge has had a noticeable impact on your academic performance and you need to explain why your grades don’t reflect your academic potential. In your case, your grades do reflect your potential - you’re in the top 10% of the class and therefore, presumably, among the most capable.
There may also be slight - even unconscious - bias against students who have a disability and are on medication. Schools are always concerned about liability in the event that a student either diesn’t maintain the med regime or their needs changed and the meds aren’t doing the job any more - especially in a new and unfamiliar environment. You don’t want to give them a reason to pause and consider the risk, even if they consciously know this isn’t a legit reason to reject you.
Thank you N’s Mom for the thorough answer! I was just making sure 
I’ll keep it a “secret” and hopefully, I will show immense academic improvement this year!