D&D setting planes

Plane travel is a part of the magic of D&D, but my setting does not have that capability.  The setting is the Proto World.  Meaning that it is the world.  It is the Prime Material Plane in whole.  There are stars and there are planets around those stars.  There may be civilizations around those stars but they better not come here, because their technology for traveling will not work in the system.  The three Sisters hold absolute power within the setting.  

There are three moons in sky, one is green, one blue, one white.  They hold a separate World of creatures.  

The white moon is the god of good, but the clouds of this world hide a dark secret of fiery pits and lofty peaks.  The fiery pits are filled with horrors and pit fiends devils and demons.  The ice fields are filled with cold loving demons and devils and with them all are managed by angels and devas.

The green moon of the Dark god hides the shadow world filled with forests and wildlife being hunted almost continuously in a cycle of life, this is where the Fae come from. The Ravenloft setting is a good place for this place too.  Fae horrors and Ravenloft horrors too.  

The blue moon of the neutral god holds a world that is filled with deep sea dangers and such horrors as Aboleth and islands with Illithids and their slave races.  

Each of the three Worlds is conected to elementally to each other a nd the main world.  Elementals pass between the worlds freely at places of elemental purity. A pure water spring, a pit of lava, a homogeneous rock face, a violent storm, or things similar.  They can be summoned through lesser versions.  Demons and devils can be summoned as well, but only through greater difficulty, as they need to bypass the guardians in the world.  Angellic guardians can be summoned too and they will also come to hunt the demonic summons.

Demons and devils are just other creatures that are natural to the world albeit more dark and twisted regions, like the deep volcanic Underdark and the towering heights in glaciers too cold.  The Demon Princes are manifestations of the God Drogath or minor creations of thought.  Balrogs are evil creatures  of fire.  Effreeti and other elemental creatures are expressions of the element in its pure form.  

Summoning creatures of power could also attract a Guardian.  Guardians view the continent as an ancestral home, even though they are from a more harsh and cruel alternate plane.  The guardians vie for the chance to prove themselves to be the best on their home plane, the best are accepted into the best schools and they train in a grueling competition, the best graduate and head out to the proving grounds that are on the edge of their society the twenty year journey tests them in ways that their schooling has prepared them for, but still only one in a thousand will survive to the end of the journey, and that means that almost all of the graduates fail, usually only one of the graduates in any ten year period make it to the end.  If they make it to the end, they pass into the world through their ancient portals into this world.  It is not hyperbole to suggest these creatures are the pinnacle of human development and can beat any single foe in one on one tests in any area.  Being truely neutral to politics, they are only interested in threats to creation.  This includes rogue members of their group and Demon Princes and potent undead.