Hey, I'm looking into The Citadel for next year as well.
From what I understand (website, and a friend who graduated from there), Cadets can graduate early. If you graduate early, you can still come back to graduate with your class. You could also just use those credits to pursue a second major or minor. Now, I don't know how the ROTC scholarship will impact that, but I'm sure you can find out by emailing the office of admissions or the registrar.
Talk to the recruiter at your ROTC detachment. Some of the services can be kind of picky about that. I knew a guy who got a four-year Army ROTC scholarship and then had enough credits to graduate in 3 1/2 years, but they wouldn't let him because he had signed the contract for the four-year scholarship.
I can't say if this is normal, and I haven't heard anything either way for the other services' ROTC scholarships. Your best bet is to talk to someone at the ROTC detachment you'll be training with.