For us, the easiest way to check for a leak in the freshwater valve is to close the seacock to the main intake strainer/sea-chest and then remove the top of the strainer and disable the head intake pumps (you can flush using water jugs of sea or fresh water or fresh water from the sink or shower in the head), AC, etc.  Note the water level (you can pull out a few inches with the toilet supply pump before disabling it).  You can continue to use fresh water in the boat.  

After a few hours and certainly by 24 hours you will see a rise in the level in the strainer.  That water is coming from the leak and the 3-way valve needs to be replaced.

A leak in the 3 way can flood your generator and possibly your engine with fresh water under certain conditions.  This happened to us at the dock and we caught it just before it flooded the generator.  We had the sea-cock to the strainer closed and were using fresh water in the heads and weren't using AC.  Because the strainer seacock is closed water will force itself  through the raw water pumps of both the generator and engine filling the lift muffler and then exhaust loop before flowing out the exhaust  Water then floods the exhaust manifold of the engine and will flood the cylinders and then make its way into the oil pan.   For some reason the water does not exit the AC system which is lower.  Probably because of resistance in the one-way valve.

The main point is that the risk of a leaking 3-way can be more than just excessive fresh water usage.  It is well worth testing this, particularly if you are at dock with the sea-cock closed.  Hearing your fresh water pump cycle on during a period of non-water usage is a good reason to check for this or other leaks.

Bob     KAIMI 429