I can't speak for ROTC admissions but (from a USNA perspective) your situation would appear to be more a "character" issue than a medical one. DODMERB deals with medical qualification. If you have not suffered injury/impairment as a result of your substance abuse and are not currently using such substances, I don't know that DODMERB would consider it a DQ -- I'll defer to a DODMERB expert on this point.
HOWEVER -- and it's a huge however -- officer accession sources definitely care about prior drug use and alcohol use/abuse. (Enlisted accession cares as well, but we're dealing with officer accession here.) They care for two reasons. First, as you undoubtedly know, the military does not tolerate drug use. Period. (Alcohol is also an issue if it involves your performance, leads to DUIs, etc.). They test and test and test for drug use and a single incident as an officer will get you thrown out -- and that's assuming you don't kill yourself or someone else in the process. So, officer accession sources worry that, if you used habitually at one time, you're more likely to return to that habit than someone who has never used. And they may not want to take that chance.
The second issue is judgment. "Experimenting" even once evidences a certain lack of judgment. However, no one is perfect and, if the "kid" was young and learned from that experience, he/she might get a second chance. Obviously, the more times one uses the less judgment that person shows.
USNA uses a "character board" to evaluate situations like yours. It is an individualized determination based on specific facts and circumstances. Not sure how ROTC handles this.
In your case, the experience was lengthy and repeated, although it did occur 3 yrs ago. OTOH, the military is downsizing and there are plenty of extremely well qualified people who don't have your history. My gut feel is you'll have a hard time.
If serving is your strong desire, you should pursue your dream. However, you should keep in mind that your past may well come back to haunt you and thus should be fully prepared with a Plan B that doesn't involve the military.