gkumar, it is perfectly OK to email all 3 (or even 9), and schedule with the one that responds the soonest or most convenient. chances are more than half of them are going to be out of town etc.
If another alumni responds after you've already set up an interview, just reply to thank them saying the instructions were for you to email multiple alumni. They understand this process can be time consuming so no one would blame you for following the instructions.
before you pick an interviewer, i would suggest you look up their profiles and pick the one (based on job, gender, age, ethnicity etc) that you think you can get along with easier.