This is from Book 2, Chapter 23 of Salamandastron. Samkin, Arula, and Spriggat are sneaking at night into a vermin camp, planning to steal back the sword of Martin the Warrior.
Under a burgeoning three-part moon they set off through the woodland, slipping silently along amid the shadowed tree-trunks and undergrowth.
I'm not sure what a "three-part moon" is. Searching brings up "three part moon phase cards" for teaching the phases of the moon, and well as everything from a "Triple Goddess" which includes the moon as an aspect to a three-part moon mission. Safely discarding the latter two results, I also doubt that the first one is relevant, since the characters are looking at a single moon - not multiple phases.
Having exhausted my admittedly limited search skills, I now turn to Literature SE. What is a "three-part moon"?