In the Viking lands, we three gods reign supreme
In the three kingdoms, where not all is as it seems.
In the frigid North, En leads the three
Just like in the South, as it should be.
In the temperate South, To makes up the center
The North agrees, but the Middle is the dissenter.
In the Middle, Tre comes in the first
South and North agree, that is certainly the worst.
We gods are amorphous, as it is well known,
Our forms may change in the domains of each zone.
In the extreme lands, En appears like Janus,
Near the Middle, however, they find that heinous.
In the Middle, To appears surprised,
Everywhere else, however, He appears incised.
Only Tre, however, is truly immutable,
Everywhere, His nature is truly indisputable.
What are we? What are the Three Kingdoms?
The puzzle hinges on the major local languages of the Scandinavian region, as well as some basic geography to determine their respective regions. While it does require some research, the correct solution will (hopefully) be obvious once it is reached.
The pseudonyms of the Gods don't actually have much bearing on the puzzle, and can be changed without affecting the answers. However, they do act as a (very minor) hint when taken in conjunction with Paragraph 2 of the riddle.