How do I remove everything Parallels?
(Optional:) backup the content of the volume "Backup". If the volume is empty don't backup anything.
Boot to Recovery Mode by pressing cmdR.
Open Terminal from the menubar -> Utilities
Enter diskutil list to get an overview.
Determine the sequence of the partitions on the RAID.
Quit Terminal and open Disk Utility

If the sequence is (1: EFI) - 2: Parallels - 3: Untitled - 4: Backup either choose the superior Apple RAID device, "Erase" and repartition the RAID disk if you don't want to preserve "Backup" or choose the superior Apple RAID device, click "Partition", choose the "Untitled" partition in the new overlay and click the "-" button to combine "Parallels" and "Untitled" if you want to preserve "Backup". In the latter case choose the enlarged partition "Parallels" and click "Erase" to create a new HFS+ file system and rename the volume.

If the sequence is different consider the following rule: If you choose a partition x (with x ≠ 1 or 2) and click the "-" button the partition will be combined with partition (x-1) and only the content of partition (x-1) will be preserved.
Quit Disk Utility and reboot to your main system.
Remove any remains of Parallels 3.0 as indicated in user3439894's comment: How to remove Parallels Desktop 3 Completely?.