If you buy a RAID card that can support RAID 0+1 it is feasible.
It might be a bit iffy but it should work no problem, but you will need an additional RAID card because motherboard do not often support RAID 0+1.
And it would work the way you are thinking, the 2x 500GB drives would make a 1TB space and then that can be in RAID 1 configuration with the 1TB drive.
You would set that up without any data on the drive, since it will erase everything, and you would, once you have the card, boot into the card (there should be an option during boot) and then tell the card what to do. Make sure to read the RAID card manual and make sure to get one that supports RAID 0+1