F12 on first time, shows first bar, u can use skills from those using F1 to F9
F12 again, enables the second skill bar and u CAN USE THE SKILLS using F1 to F9 (first bar's skills wont be used)
F12 again, for the third bar.
This was how it worked, when i joined like 4 years or something ago.
I dont know why it doesnt work anymore! i use to use all priest skills, just by swapping f12 and using f1 to f9!