Hey folks, the RAID wiki has been updated so with three disks you're spot on - it should be RAID 1 + a hot spare.
Using the GUI partitioning tool, even in auto partition, will overwrite our defaults and do a single disk install.
Note that the minimum disk size for a SME10 install is 5GB - if you're using 4GB disks that may be what's causing the 'error checking storage configuration' message.
Can you please share the output of lsblk? Thanks!