Alright, ramblings of a madman comment because I couldn't just keep quiet about how ingenious "Sangetsusei" is as a title:

Sangetsusei → 三月精

三精 (sansei) is a word (although I reckon a pretty obscure one) used to designate the three lights: the sun, the moon, and the stars (Eiki does talk about this when discussing the 60-year cycle with Reimu).

Now, I'm fairly confident that this word was the origin of the concept for the three fairies of light: partly because their names, designs and personalities are based on the aforementioned three lights, and partly because the 精 kanji is one often used to talk about spirits and it's part of 妖精 (yousei), which means "fairy".

Finally, "sangetsusei" is in and of itself another reference to the three lights. "San", as close to the English pronunciation of "sun", "getsu" is the reading and the literal kanji for "moon, and "sei" is also the reading for 星, the kanji for "star".