From Warhammer 40k - Lexicanum
The Three Planets are part of the Ultramarines realm of Ultramar. The planets are Masali, Tarentus and Quintarn. They are three small planets forming a triple world combination, orbiting around a common centre of gravity.
- 1: Gathering Storm: Rise of the Primarch, pg. 10
- 2: Codex: Ultramarines (2nd Edition), [Needs Citation]