Your disk is in MBR format. You can't use more that 2TB on MBR disk.
To use rest of that unallocated space, you'll need to convert your disk to GPT format.
Unfortunately conversion process MBR->GPT is data destructive.
You have to:
backup data from your 3TB drive,
delete all partitions,
convert disk to GPT,
restore your data.
And that's not all. You can boot from GPT disk only in UEFI boot mode.
your system must support UEFI boot,
BIOS must be configured accordingly and
you'll have to reinstall windows.
So - do you think it is worth to reclaim those 746GB of space?