Short answers, pulled from the story.
Poul Anderson published the novella version of Three Hearts and Three Lions in Fantasy & Science Fiction magazine during 1953. The story expanded into a full-length novel that appeared three years later, in 1961.
Anderson added Alianora the swan maiden and Hugi the dwarf to Holger Carlsen's original journey. They follow Duke Alfric who plots to imprison Holger inside Elf Hill where time flows differently.
The dividing line between the Holy Roman Empire and Faerie mirrors the Cold War tensions of the early 1960s. Anderson set the story against the backdrop of World War II resistance efforts near Elsinore.
Dungeons & Dragons drew heavily from its alignment system grouping creatures into Law and Chaos categories. Game designers took inspiration from Anderson's depiction of trolls whose bodies regenerate quickly after injury.
Holger appears as a minor character in Anderson's A Midsummer Tempest where he wanders alternate timelines using spells from a medieval grimoire. Harry Turtledove wrote a short story titled The Man Who Came Late in 2014 set thirty years after the original events.