At this time, our transformers (which are MvParts) do not support electrical loads. In order to accomplish what you want, you should insert a panel with (3) 3-pole circuits. This will serve as the transformer to which you'll connect the other three panels via the trough. Then, all you need to do is connect that transformer/panel to one 3-pole circuit in the distribution panel.
You can easily replace the panel blocks in plan and model to look like a transformer as well. Just insert a transformer MvPart off to the side and explode it. Update your "panel" blocks to use the transformer blocks to achieve the desired look in plan and model views and erase the exploded transformer blocks.
While not an ideal solution, it should work for your purposes. Supporting electrical loads on MvParts is a wishlist item we hope to tackle soon.
Hope that helps,
Toby Smith, PE, LEED AP
AutoCAD MEP Product Manager