Stuart - i don't think this is a fuel tap issue. I had a similar issue on my 750 k6 recently and turned out to be a crack in the brass overflow tube itself. First thing you need to do is double check the float height setting on carb 3. Also make sure the float needle seat is actually seating / sealing properly (i.e cutting the fuel supply to that carb when the float is at the correct height to close). Then try swapping float bowls with one of the carbs that isn't leaking. If the fault moves with the bowl you may have a dodgy overflow tube. If it persists then it's either a float height / operation issue (float could also be sticking) or a needle valve issue...
Also, when you see it leaking / overflowing try gently tapping the bowl with a spanner - again if it stops leaking then it's probably a sticky float.
Hope that makes sense (ish)...