I didn't have a WD 3TB drive, but we have a Raid10 with 4TB consisting of 4x 2TB WD Caviar Blue. When I tried installing proxmox on the array installation just failed during creation of the LVM-volumes. I'm afraid you will face the same problem.
My solution was: install proxmox on a smaller drive (I used a 250GB-drive), start your system with a live-cd (e.g. ubuntu 10.10-server did a good job to me!) and partition your drive manually (create a small boot partition with ext3-fs and two partitions with lvm-fs each not larger than 2TB). Afterwards you have to create the lvm-structure. Create a volume group called some different way then proxmox does, original proxmox uses pve, so at first call it pve2 or something like that. Join the physical drives to the volume group. Afterwards create the logical volumes inside, proxmox uses a swap-volume (dev/pve2/swap) of the size of your RAM, a root-volume (/dev/pve2/root, 58GB on my system) and a data-volume (/dev/pve2/data the whole remaining drive-space). When your done activate the new volume-group und format each partition (ext3 for the small boot partition e.g. /dev/sda1, /dev/pve2/root and /dev/pve2/data, swap for /dev/pve2/swap). Next step is mounting the drives and rsyncing the data from each partition of the small installation-hdd to the corresponding partitions on the new large drive. Shut down your system and disconnect the small hdd. Start the system again with the live-cd and rename the volume-group from pve2 to original proxmox-pve. Mount the partitions under /mnt or /media the way it will be mounted in a working system (/dev/pve/root -> /, /dev/sda1 -> /boot), chroot into the structure and run grub-install. Afterwards reboot without live-cd and Proxmox should be starting!