What is the difference between an Ultra 3 SCSI drive and an Ultra 320 SCSI drive?

The only difference is that Ultra 3 drive is slower in bus speed. It has a bus speed of 160MB/s. An Ultra 320 drive has a bus speed of 320MB/s.

When mixing Ultra 3 and Ultra 320 drives together, the controller will pick the slower speed of the two. If you are adding an Ultra 320 drive to a Ultra 3 RAID, make sure your server’s support the Ultra 320 drives.