Hi, Philip, and welcome to the forum!
Are you referring to the setup menu shown on page 43 of the owners manual? If so, it's possible that the "HDMI" column has a "-" in it because some other input has HDMI 3 assigned to it. Are you saying that you can't go into that column in the menu and select "3"?
In general, you can connect any HDMI source to any of those HDMI inputs. The assignments in the Marantz menu are for (a) telling it which of the jacks on the back you want to have priority (e.g. HDMI versus analog inputs), and (b) telling the receiver which source selection is associated with each jack. You can also rename your sources, such as renaming "DVD" to "Blu-Ray" (see page 44 of the manual), to make it suit your devices better. Marantz choose to label the ports with the default source settings, but you don't have to stick with those defaults.
