You need to make sure you are formatting the drive using a GPT partition and not an MBR partition to have access to the full 3TB, you may also be using 32bit copy of windows 7 which is unable to use drives with a capacity over 2TB.
Win 7 32 bit can read/write to GPT disks, they just can't boot from them.Q. Can Windows 7, Windows Vista, and Windows Server 2008 read, write, and boot from GPT disks?
A. Yes, all versions can use GPT partitioned disks for data. Booting is only supported for 64-bit editions on UEFI-based systems.http://msdn.microsoft.com/en-us/library/windows/hardware/gg463525.aspx#X-201104111922342