Yes you bring up a good point which I forgot to mention. The 3 spool valve will be a closed center. This will not let it flow any hydraulic oil until one of the spools is used.Looking great so far.
If you feed your 3 spool from the pto's PB, won't that mean that no oil flows back up to your flow control and then on to your TCV? I thought you'd have to feed your TCV from the pto's PB.
As you said if it was an open center all the hydraulic oil would just go the path of least resistance straight back to the return circuit via the 3 spool valve.
Talking to some of the engineers at the place I used to work, running the closed center should be fine. The only time it will see a high pressure would be when the tractor is really lugging down driving. Even then the the system has been upgraded to handle this pressure. So instead of the rear pto being the higher master pressure relief, it will now be the 3 spool valve. Then the rear pto valve set a little lower, then the TCV being a little lower yet.
As of right now the only foreseen problem could be is cylinder creep. Depending on the quality of the 3 spool valve, I may get a bit of cylinder creep under high pressure. This could be remedied by either a higher quality valve or a complete hydraulic shut off valve to the 3 spool.
One thing you could be correct about is which valve gets the return and power beyond on the rear PTO. I maybe have to switch them as of right now the return goes to the TCV, the power beyond goes to the 3 spool. So depending on which circuit gets priority, they maybe have to be switched so the tractor doesn't stop while using the 3 spool. I don't care if the 3 spool is a little slower as it is just for cylinders. I however do not want the tractor to stop while using the 3 spool.