DPS and Tanks:

Top bar: Role abilities and LB (plus healing and rez if the job has it)
Middle bar: Mitigation, buffs, and cooldowns (arms length goes here if playing tank)
Bottom bar: All damage abilities

Healers:

Top bar: Role abilities, rez, and any "oh god everyone is going to die unless I use this" abilities
Middle bar: Panic buttons, cooldowns, and buffs
Bottom bar: Damage abilities and core rotation healing

Following this I've been able to comfortably fit every job thusfar onto three bars.