The three Starlights are from another planet that was destroyed by Galaxia. They are on Earth searching for their princess, Princess Kakyuu (whose music you should be hearing right now), disguised as the male idol group Three Lights. The way they search for her is to sing songs calling out to her (hidden message in their songs). They go to Juuban High School, the same school as the inners and are in the same classes as them too. Everywhere they go, they are followed by drooling girls. Rei is jealous of course, since she goes to a different school. All the girls in the school are constantly flirting with them. But the Three Lights claim they could only love one person, which is their princess. Yaten actually throws away the love letters he recieves!! They are disgusted with the idea that people would claim that they love them when the don't even know them.
Chibi Chibi is a completely different story, two in fact, so you will have to go to her page and read up on it to find out. Isn't she cute? She is many people's favorite senshi. Her vocabulary is very small...it consists of "Chibi?" "Chibi Chibi?" "Chibi!" and "Chibi Chibi!" (and the double of that too, I suppose ^_-). She sometimes copies what other people say, like Ne?, Hai!, and arigatou! In the manga she actually speaks real words and phrases by herself. ^_^
Chibi Chibi is not one of the Starlights or one of the Three Lights. Some people seem to think she is, but she is not. She merely appears in the same season, Sailor Stars, as the Starlights do. If she had to be grouped, I would group her as an Inner Senshi since in the Manga she is a form of Sailor Moon.