The 3pt valve dumps to the tank unless it's engaged to raise the 3pt. That's how my Branson's BH is plumbed in. One hose comes from the loader through two sets of remote valves, into a loop with a set of quick connects and then into the back of the transmission case where the 3pt valve and cylinders are.
You can add your valve in that loop but the valve you add should have no PB (just a plug in that port) and it's tank port must be rated for full system pressure. Many valves do not have that latter feature but there are a lot that do. When you lift something with the 3pt, all the valves in the system see that pressure on their in and PB (or tank if they don't have PB) ports.
You may have to find the manufacturer's spec sheet for the valve you want to see what its tank port can handle as many retailers don't provide that info.