Looking at http://supportapj.dell.com/support/edocs/software/eslvmwre/VS_5/Docs/VMotion_Matrix/VM_mat.pdf, it appears that you can have all three of these hosts in a cluster. Putting them in a cluster would allow you to share the resources of all three servers, would allow all three servers to have access to the same shared storage, and provide more options for using HA. Putting them in a single cluster with HA would mean that the VMs could be restarted auomatically in the event of a host failure. vMotion would allow for scheduled/preventitive maintenance to occur with no downtime.
The cons of using a single host are obvious, and I wouldn't personally do that with the resources you have.