I use one button for overpower, revenge and whirlwind as they are all stance specific.
/cast [stance:1] Overpower; [stance:2] Revenge; [stance:3] Whirlwind
I think that’s right, from memory as I’m not at my PC.
There’s also a case for a button that switches your stance and casts a spell at the same time (they have separate GCDs so one press will fire them both).
/cast Battle Stance
/cast Overpower
You’ll need separate buttons for your spells though as they will change your stance.