In Windows Server 2012 Storage Spaces and Windows 8 Storage Spaces, a 2-way mirror requires at least 2 physical disks. However, a 3-way mirror requires at least 5 physical disks.
The reason is that a 3-way mirror uses a quorum. In order to keep running, the mirror space must keep over 50% of the disks functioning. So a 3-way mirror must have at least 5 physical disks to be able to survive the loss of up to 2 physical disks.
A 2-way mirror does not require this type of majority vote.