Hi Fares,
You can have only 2 servers in CUC cluster PUB and SUB. see below for details.
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/connection/11x/install_upgrade/guide/b_11xcuciumg/b_11xcuciumg_chapter_011.html
The Cisco Unity Connection cluster deployment provides high availability voice messaging through the two servers that run the same versions of Unity Connection. The first server in the cluster is the publisher server and the second server is the subscriber server.
(Rate is it helps)
JB