In reply to Sandeep.

Hi,
I think you mean you use all 3 hard disks in raid0, so you get the sum of the capacity of the drives (but if one of them dies then you lose *all* of your data).
In raid0, you can easily add an additional hard drive to get the extra spaces. Again, you risk losing all your data if one of the disks dies.
That's not so easy with other configurations (mirroring and RAIDZ) because if you add an additional drive you'd be striping it to those you had originally. Say you have a 3-disk RAIDZ1. You get the capacity of 2 disks. If you add a fourth disk you'll gain that disks' worth of capacity. But if it dies, you loose all of your data. If any of the other 3 dies, you still have your data due to the redundancy of RAIDZ.