If anyone is good at stats, I'm sure there is a way to calculate this.
On average, if you have an interview at a school, you have about a 35% to get in (each school is different though).
So...35% + 35% + 35 % = x
I think you add percentages adding the percentage of "what is left".
So, for the first interview, you have a 35% chance. There is a 65% you don't get in an interview (this is "what is left"). If you have a second interview, then, out out of that 65%, you will get in 35% of the time to that second school.
So, for two interviews, you have .35 + (.35 * (1.00 - .35) ) = .577
For three interviews: .35 + (.35 * (1.00 - .35) + .35 (1.00 - .577) = .724
For four interviews = .82. Five = .88. Six = .92. Seven = .95
Someone with more math skills than me correct me if I'm wrong.
These are just numbers though, so be careful. If you are a great interviewer or terrible interview, the numbers won't work for you.
I wish I had more interviews, but to me, getting one from the school I really want to go to is worth more than 4 or 5 from some other schools.