I am trying to visualize in my mind how your system is hooked up. Pictures would help.
First of all, does your valve look like the valve in the thread I had you look at? Is it a single lever or two lever or even possibly three??
From what I am understanding is you have a Massey valve bolted to the lift cover in front of the seat between your legs. Is this correct? You also have an additional two spool valve to operate the loader. This valve is fed from the left hand side(edit I should have said right hand side) of the lift cover to a remote, then back to the additional valve to run the loader. There should also be a return line from the other side of the additional valve back to the other remote or returning to the differential case some how.
I am guessing that you have a single Massey valve on the top of the lift cover. With the additional valve being fed from the lift cover, What you have is an open center hydraulic system. This means that oil is pumped from the main pump in the differential to the top cover thru the standpipe and since the additional valve is open center, the oil is returning to tank unable to build pressure anywhere in the system until you pull a lever for the loader and of course the loader lift or bucket operates.
This is pure speculation since I am still not 100% sure of your hookups but, if you run the position and draft lever in the transport position, pull the right hand lever to the loader and hold it, once the loader cylinders reach maximum stroke, pull the left hand lever on the Massey valve, at the same time, I think the three point will raise.
What I am thinking is by holding the right hand lever for the loader, once the cylinders reach maximum stroke, it will block oil from returning to tank and thus build pressure in the three point circuit.
If this works, you could add a shuttle valve blocking oil from the loader circuit when you want to use the three point or feed the loader valve from the right hand side of your Massey valve if it is two spool. You could disconnect the hose to the loader valve and cap it and plug the lift cover and see if the three point operates properly. Good luck