I also have a 2-way valve in my fresh water flush line for the watermaker. (actually two...) that is closed all the time unless I am actually flushing with fresh water. I might very well have a leaking 3-way valve, and will never know...
I am assuming if you close the 2-way valve in your freshwater flush line the leaking 3-way valve would then have no symptoms. So my first reaction is, what's to fix? But... if you regularly flush the watermaker and the main seachest seacock stays closed, you could eventually overfill the raw water circuit and flood engine exhaust, so a fix might be a good idea anyway.
There are only three failure modes I can think of for this valve: corrosion of the body, corrosion of the ball, or failure of the seats. None of these are an easy fix. The valve has shown itself prone to failure, new valves are not very expensive. Why even try to fix?
Three-way valves like this should not fail this way. If I ever need to replace mine, I would not consider replacing it "in-kind", but with a more reliable valve. Bronze, 3-way, L-port, ball valves with teflon seats in an appropriate size are easily available. With a stainless ball and stainless handle, and you're all set for less than US$100. It will likely last longer than the hull.