I could only partition a 750GB part.
Lets stop here because this is where your problem starts. The actual drive is probably not 3TiB but 'marketing 3TB'. That is about the same as 2TiB+750GiB.
Your old OS looked at the drive and had a wraparound. Think of that as having two boxes to write numbers in and then you count to 120. It would start like this:
1 (first number)
Rather than wrapping at 100 your OS wrapped at 2TiB. It has a wrong idea about the drive.
"Good enough" tought I, "I can partition the rest later with Windows 8".
Turns out Win8 sees the partition as a RAW partition and cannot access it.
Windows 8 (or any OS not limited to MBR) will see the drive as it actual is. It has the right picture which sadly differs from what your XP installation thinks of the drive.
Question is, how do I recover the partition? I'm already using about 500GB
of space so backuping is the least I would like to do.
Backupping and repartitioning is exactly what I would do.
Whatever you decide to do, make sure you have a backup first.
And test that backup.
Even if it is just in case you break anything while trying to find a different solution. But if you already have a working backup then the quickest way is wipe the disk (start, run, diskpart and then clean ON THE RIGHT DRIVE) and then to repartition it.