Duncan wrote a nice article about cluster sizes with vSphere 5.0 (with a lot of interessting comments):
http://www.yellow-bricks.com/2012/04/10/cluster-sizes-vsphere-5-style/
If you would create 3 clusters keep in mind that you need to provide 3 failover hosts minimum. In your 13 node cluster you are may fine with a 12 + 1 setup but with 3 cluster setup it must be three times n + 1.
Regards,
Mario