What version of SBS are you installing? RAID 5 with 3x1TB disks will give you a 2TB "disk" available to the OS, which should be fine. Have you tried configuring RAID in CTRL-R, then skipping that step of SBUU/SMTD? Have you tried installing directly from the Windows CD/DVD? What version of SBUU/SMTD are you using?
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