I know this is old [and perhaps solved] but I'm posting my solution on a few of these "New 3TB drive showing as 750GB" threads because I never found this specific [super easy] fix posted anywhere.
Updating the ATA Controller driver via Device Manager was my first idea but it always said "up to date" so I tried many many other things until coming back to the driver. As it turned out, the latest RST driver from Intel [13 something] was not compatible with my ATA controller chipset [Asus P8P67 Pro MB, Win 7 x64, with version 10 Intel RST driver] so it wouldn't self-install/self-update. Intel chat ended up providing a link to a legacy version of RST [12 something] and installing it solved the issue immediately.
The link to the Intel RST Driver that worked for me is provided below. The SetupRST.exe from there is all I used. Good luck.https://downloadcenter.intel.com/Detail_Desc.aspx?DwnldID=23496&lang=eng