Welcome to the community.
You can add the 3rd host(my understanding you want to add the HP host to vcenter).you can use the internal HDD of the HP host for its purpose only(cant be shared across the other host).
Common shared storage is required to utlize the below purpose.
VMOTION
SVMOTION
HA
Since the HP host has only internal HDD the VM that are going reside in HP server can do VMotion(use DRS automated feature) nor HA feature VMs that going to be in the internal HDD of HP server, wont be able to see by DELL HOST.
But still the advantage of adding to vcenter is, we can configure and manage all 3 server from one console. provision VM from one template etc.
Hope this clarifies.
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