I've heard of a few people trying the 3 90-minute schedule, and I remember it was extremely hard for them, and they eventually stopped. I'd suggest an Everyman schedule. Take a 3 or 4.5 hour core with three 20-25 minute naps.
It's much more flexible than most schedules, especially compared to the 3 90-minute schedule (I'd hate to miss a nap with that!)
For puredoxyk's (the one who basically brought polyphasic sleep to the masses) opinion on schedules, go here: Paradoxes! - Everyman