Have you looked into cloud storage for a backup option ? With this kind of size (200 TB ) I would think you would be looking someplace other than a forum for help.
Is your question about replacing the 3 TB drives ? Is the number correct that your looking for 80 3 TB drives ?
If that is correct then I would consider the new WD red drives which are designed for NAS.http://www.newegg.com/Product/Product.aspx?Item=N82E16822236344
Depending on your budget the WD Black drive is one of the better all around drives and it comes in the 4 TB size now.http://www.newegg.com/Product/Product.aspx?Item=N82E16822236407