OP has confirmed they're 12TB each, and in total there is 19TB of data across the two drives.
Assuming there is only one partition, each one might look something like this:
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: 12345678-9abc-def0-1234-56789abcdef0
Device Start End Sectors Size Type
/dev/sda1 2048 23437499966 23437497919 12.0T Linux filesystem
OP wants to buy a new drive (also 12TB) and make a RAID5 array without losing existing data. Kind of madness, but it is achievable. OP buys a new drive, and set it up as such:
Device Start End Sectors Size Type
/dev/sdc1 2048 3906252047 3906250000 2.0T Linux RAID
Unallocated space:
3906252048 23437500000 19531247953 10.0T
Then, OP must shrink the existing partition to something smaller, say 10TB for example, and then make use of the rest of the space as part of their RAID5 :
Device Start End Sectors Size Type
/dev/sda1 2048 19531250000 19531247953 10.0T Linux filesystem
/dev/sda2 19531250001 23437499999 3906250000 2.0T Linux RAID
Now with the 3x 2TB partitions, they can create their RAID5 initially:
Make ext4 partition on md0, copy 4TB of data (2TB from sda1 and 2TB from sdb1) into it, verify RAID5 working properly.
Once OP is happy with the data on md0, they can delete the copied data from sda1 and sdb1, shrink the filesystem there (resize2fs), expand sda2 and sdb2, expand the sdc1, and resize the raid (mdadm --grow ...)
Rinse and repeat, at the end of the process, they'd end up having all their data in the newly created md0, which is a RAID5 volume spanning across all three disks.
Hope this is clear enough and that there is no more disconnect.