Historical Data Point 12 mentions 3 large arks being launched from their homeworld, so it would make sense with 3 sites.
Seeing as the Guardians also need to breathe, and their similarities to humans, and the fact that the planet is within the human habitable zone, its fair to say it used to have an atmosphere once, a long time ago. It is ancient after all.
So maybe thats a clue on where to look for the 2 other sites?
Landable planet in the habitable zone of an F-star, within range of an electromagnetic slingshot, possibly with a moon orbiting it.
Thats still ALOT of surface ground to cover.
You can narrow that down a bit, we know that northerners have redder skin, suggesting the planet may have different exposure to its star at either pole. This implies both an axial tilt AND tidal locking in order to increase the exposure and to stop seasonal variation balancing it back out.