I don't have CC 3, so I can't tell you about that one.
CC 8 is all hills and treks (kind of like a fast-paced, low-grade hill), but no sprints at all. It's fabulous, especially if you use an elliptical. The challenges are much longer than his other workouts. It's currently my favorite simply because I love hills.
CC 7 is a bit more traditional, with a combination of hills and sprints. The first challenge is pretty long and is out-of-this-world wonderful! There are no level 4s on it at all, which is nice. He really makes you work hard though. Trust me.
I like the music in CC 7 better because it's more lively. But even though the music in 8 is slower paced, don't let it fool you. It is excellent for the workout and really makes you get into those hard climbs.
Sean gives a full description of each workout on his Web site, broken down by each challenge. Just click on each workout for more info.
Let's see. If you don't do a lot of hills, I'd do CC 7 before doing 8. Oh, and I do know that CC 3 is shorter, so if you prefer a shorter workout today, do that one!