I think that you would be easily able to show graduating in 3-years as a strength. 15 credits isn't always easy -- 2 out of every 3 students take at least six-years to complete their undergraduate degrees. Doing it in less shows dedication, intelligence, work-ethic, and (in my mind) is VERY impressive. (Also, it would essentially decrease any students loans by 25% -- now that the average college student graduates with $30 in debt that's about a savings of $7,000.)
The only drawbacks that I'm aware of would involve missing chances to build a resume. However, if you stay on top of things you could easily complete them all.
1) Rushing through school so quickly that you do not get involved with student organizations:
They're big resume builders and will give you hours of volunteer work and exposure to professionals and persons who may be the gateway to finding a great job. Look into joining and leading those clubs. Being President/Secretary isn't as hard as you think and probably takes 3-4 hours a week, but adds up to over 240 hours before you graduate.
2) Never completing any internships, or completing the bare minimum "easy" internship:
Sure, unpaid internships suck, find a paid one if possible. Get several. They'll add to your experience, add to your resume and, in many cases be the difference of several thousand dollars a year when it comes to your salary. (If you're going to earn a Master's this is slightly less important but experience can still be a big factor that shines through during the interview process)
Anecdotally, I deferred one semester when the right paid internship came a long. I was able to use the paid internship as a spring-board to complete other part-time work while I was at school that really enriched my college experience. I've never regretted that decision.
3) Never becoming "chummy" with any of your professors:
Several reasons: these guys/girls are often the smartest people you'll meet. The best possible job you can get on campus will be with them as a grader, lab assistant, etc. They're chuck full of good advice and have chosen to share it for a living. They are undoubtably the best, and most underutilized resource on campus. Sure, there are bad apples in the bunch but you'll meet many great professors in school. (If you're going to graduate school, they'll be able to write much better letters of recommendation if they know you.)
Best of luck!
Comment, or message me if you have any further questions. Best of luck to you!