I agree, college can be a drain on barn time. I'm starting my sophomore year classes on Monday, and I consider myself very lucky to be able to keep my horse (many thanks to my sister, who takes care of him for me). I go to a small school about an hour from home, but it's a residential school, so I live on campus. I am able to come home most weekends, and I have a nice, long Christmas break, but if it werent for my sis, I don't know what I'd do with Scout.
I'm fuzzy on the 3 horses you already have. Do your parents not ride? Do they want to keep these 3, or do they not want to be bothered with the hassle of marketing and selling them? 3 "green broke" horses is a lot for any recreational owner. If I were in your situation (granted, with my parents... and my horse-lovin', awesome sister to help), I would consider selling at least the least trained of the current horses (I don't know what your situation is this way, but in my family the horses are kinda my project. If I wanted to sell mine, I'd be asked for a reason why, and then told that it's my job to handle it), and take a look around for an older, seasoned trail horse. $350 is a darn good price, but it is low. Could be the economy forcing a low price, it's definitely a buyer's market right now, but there could be serious issues with the horse. Look close, get a vet check.
This is a touchy thing, be sure of what your physical capabilities are. I sound like my dad right now, but College Comes First, Horses Are a Bonus. Don't stretch yourself too thin, it runs you down and won't do you any good at study time.
Hope that helps a bit, best of luck with your situation and with classes!