You NEVER want to block the output of your water
maker product water when the unit is running. You need to use a Non-blocking or transitioning 3-way valve or you run the risk of blowing apart your low pressure plumbing
if you block the product water. You can also damage your membrane by causing abrupt pressure fluxuations when turning the valve. Or I have seen the valve inadvertantly bumbed in the middle "off" position and POP goes the low pressure line and things around there get wet....hopefully not expensive electronics
If you can't find a non-blocking 3-way that will not block the product water flow when turning positions, then as a water maker manufacturer, I would recommed you use two on/off valves and a T fitting. Always OPEN them both and then close the one you want to shut off.
If you blow your low pressure plumbing
or damage your membrane by using the improper 3-way non-blocking valve...well you don't want to risk that.
The valve Mark listed on grainger:
SMC CORP. OF AMERICA/SMC PVC Ball Valve,3-Way,FNPT x FNPT,1/4 In - Ball Valves - 3CGZ9|2362890 - Grainger Industrial Supply
Is a great valve and also comes in a nice clean panel mount option and is used by us and sheesh...I think most of the water maker companies for their inlet water selection...but using the non-blocking 3-way on the product water outlet plumbing is a No-no.