While the specific voices of the girls aren't credited, it's likely that it's a combination of the voice actresses who played the girls during that time period. The most common voices for the girls were:
* Blossom: Cathy Cavadini
* Bubbles: Tara Strong
* Buttercup: E.G. Daily
So, while you may not be able to identify one single singer, it's safe to say that the end theme song is sung by the girls themselves!