If you are done taking whatever these classes are, the first thing I would think about is how often (if ever) you might view this material in the future. I would weed out things you don't need, before spending a lot of money on storage for something that will take up a lot of space and be seldom used. Even storing electronically involves the cost to convert it and whatever means you use you have to organize it such that you can easily locate what you are looking for. Having to look in 5 boxes to find 1 document would certainly waste a lot of time.
Even while I was in college, I RARELY (if ever) needed to refer to something from a prior class, regardless of whether it was some textbook or term paper I wrote. Eventually, I just tossed all of that material in the trash and have never had a need for it.
I assume when you refer to things in a 3-ring binder, these are probably old term papers/tests or various class handouts. What field are you in where you would ever need to refer back to these?