|Does the drive show full capacity in BIOS?|
Does the drive show full capacity in Windows Disk Manager?
What file system did you use when you formatted the drive?
Is the drive GPT or MBR?
What is the default block size of the drive?
Did you perform a 'Quick Format' or a full format?
Are you sure you didn't create a second partition on accident?
3 TB is actually 2.75 TB, so if you had a 2 TB partition this could leave the 746 GB.
User Info Page Biography for full specs.