I honestly don't know. Most of the Yuri 日本动漫 I've seen, I've seen through porn 视频 (yes, I watch porn. Problem?), most of which don't state the name of the anime.
I guess La Blue Girl. It's not EXACTLY yuri (haven't had a chance to see the whole thing), but I do remember seeing a threesome scene between Miko and two other characters. Plus, I actually KNOW the name of this series.