Thanks for the screenshot.
As you can see, the 3TB disk is currently only used partially. Did you intentionally limit the size during the installation, or did you increase it after the installation?
Can you post the output of
lsblk
as that would give a more complete picture?
Depending on what you actually want, you will most likely need to increase the 3rd partition, or add another partition to make use of the 3TB volume. You will also need to configure a storage on /dev/sda. Could be a directory or LVM, LVM thin if you want to use snapshots with LVM.
If you upload files via the GUI, it will temporarily be stored in the /tmp directory which is located in the root FS. Therefore, that won't work well for very large files in this situation where the root FS is not large enough.
If you want to import those vmdk files to be used for VMs, consider accessing them directly where they are (network share?). Create the VM (without disks) as you need it, use the
qm importdisk
(
manual page) command. It will convert the disk image to the correct format on the fly and store the result already in the target storage that you choose for that VM. This saves you from having the disk image around twice before you import it to the VM.