Pulled up a picture of the motherboard from ASUS, seems like you have three fan/AIO headers up top. AIO pump at rightmost, then CPU_FAN, and CPU_OPT at the leftmost. You can plug the AIO pump (3-pin connector), and the fans to CPU_FAN and CPU_OPT. If you need more, you can always get a splitter or ideally a powered fan hub.
The pump will always run at full speed if the header it's plugged into is configured to use PWM control, otherwise you need voltage (or DC) mode to control the pump. Not sure about 280EX, but my Captain 120EX's pump is silent even when I run it at full speed.
Is your AIO pump plugged in to the correct header? If so, I suppose you can just use DC mode if available in the BIOS to control speed, otherwise I'd leave it at full speed and let the fans ramp up and down accordingly. If there's still error, I see no harm in just turning off the warning. Your pump is running, the fans are running, and if temps are within expectations then no harm no foul. I'd argue that 3-pin AIO pumps are meant to run at full speed all the time anyway.