Plug a 12TB Seagate into the SATA data and SATA power cables currently connected to the 10TB drive.
If the 12TB drive is recognised on the 10TB cables, check the SATA ports on the mobo for the 12TB drives and see if they're disabled by an M.2 drive or PCIe card.
Due to the limited number of CPU/chipset lanes on some motherboards, you have to choose between extra SATA drives, or another M.2 NVMe drive, or an auxiliary PCIe card.
TLDR. The SATA ports for your 12TB drives might not be active.
I do hope they're not ST12000NM0007 drives. Backblaze had an 11.77% failure rate with this model. I was tempted by a recent Amazon offer, but decided it wasn't worth the risk.
https://www.backblaze.com/blog/backblaze-drive-stats-for-q3-2024/
If you already have a Corsair Shift 1200W PSU as shown in your sig, I don't understand the question.
I'd probably use an 850W PSU with this configuration, or maybe 1000W. The recommended PSU rating for an RTX 4080 seems to be 750W or larger.
SATA hard disks typically pull 5 to 10W when active (3.5") and less power when idle, so 3 drives have negligible effect on the PSU.
I'm assuming you've plugged a SATA power cable into each 12TB drive. You can plug all three drives into the same PSU lead if it has enough connections.