740GB is very close to the number we expect when a > 2 TiB disk is addressed by a system that only supports 32-bit LBAs:
Your 3 TB disk has about 3E12 / 512 = 5,859,375,000 = 0x15D3EF798 LBAs. I say "about" because almost all drives provide just a little more than the advertised capacity. (Not nearly enough to get up to 3 TiB, though!) Windows will display this with a binary "T", meaning 1024 to the fourth instead of 1000 to the fourth, so it will say it is about 2.73 TB. (It really should say 2.73 TiB, or 3 TB, but that is another discussion...)
Now... if something in the chain only supports 32-bit LBAs, the number of LBAs becomes 0x5D3EF798 = 1,564,407,704 decimal LBAs. At 512 bytes/block that is about 801 GB (decimal G).
But Windows displays disk and file sizes using binary prefixes (as it does RAM sizes), so that is displayed as 746 "GB" (should be displayed as 746 GiB). Or maybe 745.96 GiB.
I originally had this as a comment to the question, not an answer... but I needed more than 500 characters to add some additional detail. This is not an "answer" as it does not solve your problem, but it may suggest a next step. I can't explain the "progressive" decline, though.