When replacing O rings of Janfy 3 way valve at the suction side, is necessary to turn off pool water supply?
If the water level is above the Jandy valve then yes, you would want to shut off the water before it reaches the valve so you do not have water backflowing at the valve once it is taken apart. If the Jandy valve is above the water level then no, you will not need to shut off the water supply.