Welcome to the Forums,
lets answer your question one by one.
When running DRM Cluster, what exactly gets shared? CPU AND RAM?
It is Actually DRS, and nothing actually gets shared, it gets pooled, a single host only has the finite resources assigned to it, but the 3 hosts together will work in unison to place your VM's in the most optimal location for performance.
How do you connect ESXI Hoster in a cluster. Gigabit NICS?
You will need a copy of vCenter, this manages the 3 hosts. you create your Cluster in there and select DRS and HA.
What software would I need to be able to run such a cluster? I beleieve that the basic free esxi wont do this?
You will need at least vSphere Essentials or higher. if not going with Essentials you will need 3 x (insert your version here together with a copy of vCenter server.
When running a cluster, does VMs site on a specific host or "on both?
Not too certain what if you mean where do the VM's sit, than the is based on the DRS algorithm. and they can move if needed
If using pass-though on certain devices, can you attach only to VM on the physical Host?
if you are using phyiscal passthrough your VM's will be tied to the host that hold the device being passed thorugh
Lets say I have 5 HDD on each host, can they be seen collectively between the ALL HOST VMs or only on the VMs hosted on that host?
not without a VSA
Thanks very much.