Why does my hard drive appear smaller than its capacity?

Bring up Disk Management and look at the partitions on the drive.  Chances are it has been previously formatted and a partition remains on it.

It's also possible that it's a used drive with defects and somebody did a high cut on it to hide the defective area, but this is exceedingly unlikely.

If you are using win7 OS, just use disk management to change 3T hard disk to GPT.
If it's MBR, system will split it as 2T and 745GB due to limitation.

I am using Windows 10 Pro 64-bit.   Does this make a difference?

Your question, suggests you connected it elsewhere and saw a different result.

Is it connecting via A sata port, or is it a USB connected external drive?

diskpart is another way, device management to see what, how the drive is recognized.

post a screenshot of disk management,
here's how to check if the drive is gpt :
Locate the disk you want to check in the Disk Management window.
Right-click it and select “Properties.” Click over to the “Volumes” tab.
To the right of “Partition style,” you'll see either “Master Boot Record (MBR)” or “GUID Partition Table (GPT),” depending on which the disk is using

The external drive enclosure was the problem.   I connected the drive to the SATA connection in another computer, extended the volume and now I have the entire disk available.

Thanks to all.

Glad you got this working!