It varies from school to school, but I'm pretty sure they interview about 5x the amount that end up enrolling, and a lot of schools accept significantly more than the final enrollment. Id ball park it at 1/3 interviewees are accepted so 3 interviews would statistically mean you should get in somewhere. I'm literally pulling these numbers out of my a$$ though so you'll need to look at the ADEA guide to get official stats of interviews vs enrollees at whatever schools you're interested in.