Testing iSCSI storage could be a bit tricker than FC SAN storage because users often fell into network configuration problem.
I think you can try both 1 host-1 datastore and 3 host-3 datastores tests since iSCSI datastores are already shared across hosts.
For maximum IOPS tests, create 9 VMDKs (3 VMDK for each datastores) and attach each VMDK from each datastore for each VM on each host.
In short, since iSCSI storage are shared across hosts, either way should be fine.
