7-night cruises are typically 1231231 (though sometimes there are variations). So you'll get two nights in each MDR, with their primary menu one night and secondary menu the next. For the other MDR, you'll get a third night there, which will be a shared menu across all three MDRs (typically this is the Pirate Night menu for Caribbean sailings).
As PP said, you won't know your specific rotation (which MDR you start in) until you get onboard.