The Arch WIKI (
https://wiki.archlinux.org/index.php...c_instructions) was very helpful.
Following its advice I used cgdisk to create a 1007 KiB type ef02 "BIOS Boot Partition" at the start of a re-initialised 3 TB HDD.
Then installed Ubuntu 12.04 32-bit. It didn't boot so, following a suggestion on the linked Arch WIKI page, I used cfdisk to mark the only partition listed (FS Type: GPT) as bootable. That may not have been necessary if I had used the installer to create the "BIOS Boot Partition".
Having proved that the BIOS+GPT+Ubuntu-12.04-32-bit combination is possible, the next step is to upgrade our in-house server with a pair of 3 TB HDDs. Any useful information gained will be posted here.