According to the official site, Hymnnos Tilia is KOKIA,
so she should be on the credits. Her name does not appear in the cd page, though:
http://www.team-e.co.jp/sp/artonelico3/st.html
Information about Tilia's character in the game is new (last four days, it was like a surprise, I guess); maybe that's why not all info is there in the CD page.
I'm a bit confused about who actually performs for Tilia, since I would swear I hear KOKIA in THE PREVIEWS:
check the HP for
http://ar-tonelico.jp/at3/, then SPECIAL, then promotional video II, there's a preview for 光の中に (it will be in the blue side?) and at the end, when they explain the photobook, I think the first voice you hear is KOKIA too!
You can preview
謳う丘~Harmonics TILIA~ (I would say that's KOKIA),
COSMOFLIPS and
REBIRTHIA, in the same page in te Hymnnos mix video.