Should I address my GPA declination in my personal statement?
Triple-majored and still above 3.5? That's very commendable. Typically when you refer to declines in academic perfomance, it's because your GPA is STILL low, perhaps below the minimum requirement for graduate school. But you seem to have graduated with an overall great GPA, and you should focus your personal statement instead on what you've learnt and done, such as research, projects, jobs - instead of that one year where things didn't go well. I'm sure you have a lot to talk about as it is.

EDIT: While a good GPA is necessary, your skill set and recommendations from your superiors you worked with are also as important. You need to show the school you're applying to what you can bring your field, which is what a personal statement is there for in the first place.