I'm pretty sure I can configure all 3 lines of the display on my 789(?) - I know I can set them all in Sirius mode. I don't have my DMS in this car right now, so I can't test it with the phatbox at the moment.
One thing to note is that you don't choose which tags to use, per se. IIRC, in PMM you select what the phatbox outputs as the album and song tags, so it's entirely possible that only two tags are available at the headunit. Some experimentation would be in order.