Before Galaxia started collecting the sailor crystals of our home system, she attacked many other systems and stole the sailor crystals of their guardian senshi. One of those star systems was the Kinmoku system, in which the planet Kinmoku orbited the star Kinmoku. (Both of these can be written as "Kinmoku-sei.")
The guardian senshi of the Kinmoku system was an imperial princess named Kakyuu; she was protected by three senshi, the SailorStarlights. (Just as Sailormoon is protected by the inner and outer senshi.) Specifically, StarFighter, StarMaker, and StarHealer. When Galaxia attacked Kinmoku, Kakyuu was sorely injured; somehow, she escaped and retreated into a chalice to rest and recover. This chalice was guarded by the not-inconsiderable abilities of a young toddler, named (quite aptly) Chibi-chibi. (Literally, "very very small.")
The three Starlight senshi were also able to escape alive, although their home system was destroyed. Seiya, Taiki, and Yaten promptly set out to find their princess, needing to protect her from further harm. (They knew that they couldn't run from Galaxia forever.) They knew that she had escaped to Earth, and so they went seeking her there. The power of song linked them to their princess, and so they posed as a singing group on Earth. They disguised themselves as males, and called their singing group the Three Lights. (Note: In the manga, they were never actually male. In the anime, they were.)
The Three Lights were a huge sensation and became idols quite quickly. In fact, girls flocked from miles around to hear them sing. Unfortunately, none of these girls was Kakyuu. Some of them, though, we know better as Sailor senshi... Aino Minako was a particularly big fan, as was Kino Makoto...
Anyway. It turns out that the chalice holding Kakyuu was actually sitting in Usagi's living room, and boy, were the Starlights surprised to find her there. When Kakyuu emerged, rested and recovered, she and the Starlights were able to aid Usagi in her fight.
Ready to meet the Kinmoku-sei senshi?
By far the best Starlights site I've seen yet: