As I understand it, the 3ph is an "open centre" type valve, so it won't build any substantial pressure at the hydraulic block (ie: the 3ph valve will be constantly bleeding off the pressure.) I believe the plug is just there to cap off the cross-drilled passage after manufacture.
The replacement hydraulic block cover has different passages to route the fluid in series. If the auxiliary hydraulics are not in actual use, then fluid is passed to the 3ph via the "power beyond" port. However, while the auxiliary hydraulics are being used, the 3ph gets no fluid, it returns to the transmission case.
Disclaimer: I could be wrong on a detail here. I have not done this yet, just been researching it and buying parts based on the research. Also, your fluid return options may be different. Mine is a B7100 and the only available return is through the fill opening in the top cover.