So I recently found out that I have sh*t my leg... I was about to set up unraid but just before dipping my toes I was told that the parity drive needs to be the same or larger in size, really wish I had done more thorough reseach before recently buying that 10TB HDD hehe...
With only 3TB in the pool you're going to be increasing the size of the pool potentially quite quickly depending on if you're ripping DVDs or Blu-Rays to it.
You'll soon need another drive to boost that pool size and I promise, that parity will keep those tears at bay if something happens.