Looking at the shipping spec of this Dell (using the service tag) it looks like it had a Dell PERC 6/i 6i PCI-e SAS RAID Controller fitted. In my experience, reconstruction of any RAIDs in EnCase that used company bespoke controllers can be very painful. Normally, I would take a logical in these instances but I see you are unable to boot the system to OS. Have you considered Linux boot disk (DEFT) with the disks fitted and logically acquire that way (and keeping fingers crossed that DEFT supports the Dell RAID controller)?
Regards
I don't know. I tried to read all three of these disks through EnCase as a preview and I didn't see anything that looked like an OS.
Sure, you won't find on any of them "anything that looked like an OS" unless they area a set of mirrored drives, which indirectly confirms that the most probable setup with three disks (a RAID 5) has been used.
Still one (and one only) of the disks should have as first sector the MBR (the first disk), which is something that you can easily check with a hex editor.
Even if it is not the first sector, a MBR must be present on first disk "near" the begiining.
If you carve the RAW disk for the Magic Bytes 55AA as last two bytes of a sector you should be able to find it in no time.
You reported earlier that TWO of the disks started with a MBR, which is the part that "sounds strange".
A number of specialized "automagic" or "autosensing" specialized tools were already recommended, personally I would have a (more "manual" ) try with DMDE which has a "raid reconstructor" that accepts a virtual reconstruction with the several possible parameters
http//dmde.com/
If you can identify first disk image, you just try adding the other two disk images, or one of the other two disk images and a NUL device, then play a bit with the possible parameters until you find something "making sense".
If it doesn't work, you then try again exchanging the two non-first disks and the non-first disk and the NUL device.
After all, even doing it "blind" or "random" it is a finite number of attempts.
jaclaz