Unable to use disk removed from Synology NAS with a Mac - wrong partition size showing
This usually happens for one of two reasons: Either your partition table is incorrect/corrupted, or you have a limiting disk interface converter connecting.

Corrupted partition table

You'll need to wipe the partition table and write a new one, which will then allow you to use the full disk.

Follow the instructions below the heading "Erase and repartition a device" on this link to do so:

https://support.apple.com/kb/ph22240?locale=en_US

If you want to be absolutely sure that you have wiped the partition table, then choose to make it an MBR table first, and then do it again and choose GUID the second time.

Limiting disk interface converter

Some old USB to SATA adapters actually limit what drive size they will report over USB. This is because they convey the number of sectors on the drive as a 32-bit number only. Your disk (3 TB) requires 36-bits to describe the number of sectors. This would make the drive report as approx. 750-800 GB instead of the correct size.

You'll need to use a different adapter to connect the disk to solve this.