Yeah, I did always find that weird. However, separate Bubbles & Buttercup articles might not be the solution here. Information about the three Powerpuff Girls individually is hard to come by within what Wikipedia considers "reliable" sources.

Something I've noticed about the Blossom article on Wikipedia is that a lot of the sources it cites talk about the Powerpuff Girls collectively, not Blossom specifically. It would be a better move if Blossom's article were rewritten & renamed to be about the Powerpuff Girls collectively. I've been a Wikipedia editor for quite some time, so I'll see about bringing attention to this issue on that article's Talk page.

As for why Bliss gets her own article... I have no idea. She's not as notable culturally as the three original Powerpuffs are. That one could actually be a candidate for deletion, especially if the idea for a collective article for the Girls is well-received (in which case info about Bliss could instead be merged there).