This is easy enough, simply make sure you have the overlord banner and purchase 2 of the other options for each mistress, one in each area (ie one in the bedchamber, one in the Throne room). To get the options to purchase items for each mistress you have to change who is your “first mistress”. To do this take the long thin corridor on the left of your bedchambers to a room where all three mistresses are, then press in front of which you want to be your first mistress. The place to change your tower decoration is also in the bedchamber.
Items in the Bedchamber: Bed, Curtains, Candles
Items in the main Chamber: Throne, Statues, Alcoves