Welcome aboard BiggarBX25.
As with most all SCUTs & CUTs, the 3ph is the lifting mechanism for the MMM. There is a linkage rod that is connected to one of the upper 3ph arms that connects to one of the MMM's lift arm. This is how the 3ph lever controls the MMM's elevation.
On the back of your BX25, near the top of the subframe that the BH connects to, there are two eye-hole brackets on either side. If you want your lower 3ph arms to stay stationary, you disconnect the vertical lift rods from the upper 3ph arms and pin them into those eye-hole brackets.
My BX23 owners manual makes mention of this in the section where it explains the operation of the 3ph so I'd say the BX25 manual should also have a short mention too.