For 3 hour long runs, you are going to need a heck of a lot more calories than you'll get with the Nuun, that's for sure! Nuun can be great for the electrolyte side of things - definitely look into that to see if it works for you. Typically, it's the premixed sports drinks that have HFCS. If you get a powdered drink mix you'll eliminate that problem. Generally, what this means is you need to go to a running, camping or cycling store (or shop online at a sports store). Not Target.
I like CarboPro - it's a tasteless powder source of readily available carbs that you mix into whatever you happen to like for your hydration, and it's great for people who don't do well with gels or solid calories. I use Gu20 for electrolytes, and mix in the amount of CarboPro per bottle that I know I need. For endurance training over 1 hour, a general rule of thumb is to consume 300 cals / hour. You'll have to fiddle with that to see what your ideal number is - but it's a good starting point.