Ok, tonight I installed the Y valves. Found out that the purchased hose will not fit the valves and fittings ordered, both are 1" :-(
One Y valve (KGB/bow bags) is now mounted between the two cup holders on the right side of the drivers seat, the other two (rear tanks/rear bags) where mounted in the left respective right corner of the rear lockers.
The plan is to use one pump to fill drain KGB/bow bags, one pump for the starboard and one for the port bag (I don't plan on using the rear tanks often). That is the reason why I decided to have one Y valve installed near the driver, that way I can easely switch between KGB and bow bags since these two I plan to use on a regular basis.