No matter how hard you work, I don't think 3 weeks will be enough to get you in shape for hockey, especially if you haven't worked out in awhile. You can try to get out there and log some miles, and do some sprints/agility stuff, and maybe try to lift a bit as well, but I think you would need at least a few months to get in shape for that.
Edit: and I will add, if you just want to increase your strength/cardio in general, I would recommend just getting out there and running a few miles a day or whatever, and trying to make use of the weight room if your college has one. Get up there a few times a week (3-4), and work out lower body, upper body, triceps, shoulders, etc. and just try to variate your workout every day. It sounds like you just want to get in good, overall shape, so just try to work on everything and gradually you'll catch up to where you once were.
"Oh, that Lankford and McGee, the trio of 'em. They're a one-man wrecking crew."