1) Pretty sure that Raid 0 you got there is probably only 2TB max. Must use the Same Size hard drives in ALL versions of raid or it will use the smallest size.
2) No you can't make a Raid 1 of a Raid 0. You have to create a Raid 0+1 to do what you want to do (Make a Mirror of a Raid 0 where as a Raid 10 is a Raid 0 of a bunch of Raid's 1's yea the number are what you think would be backwords but they aren't. The first number is the number of the LOWEST raid in the raid set)
3) To do a Raid 0+1 you need Or a Raid 10 you need at LEAST 4 hard drives of the SAME size or it will default to the smallest size hard drive AND you need a Raid that supports RAID 0+1 or Raid 10
you can keep your raid 0 and in software have your computer automatically back up everything from your raid to your 3tb. not sure if a product will let it be bootable easily though. I have never tried.
Didn't think of that. You might be able to do a Software Raid 1 though windows. But again i'm pretty sure it requires you to erase all data off the drives. This is the part that sucks about most Raids. Only High end Raid cards allow you to Expand without deleting everything.
That or as Farengottu said use a backup/Sync/Cloning program