The error is telling you that you have not set the pins to work as input and, when you try to access them as so, it fails. I had a similar problem and as far as I see it it should work (you are setting the pins after all).
Try changing GPIO.setmode(GPIO.BCM)
to GPIO.setmode(GPIO.BOARD)
. You will also have to change the pin numbers to the physical ones (yours would be 15, 16 and 18).
I still don't know why, but it did the trick on my code.