That's not how it works? If you were to Raid 0 them, it would be 120*3, not 120*2+250 as all drives have to be the same size meaning the capacity on a bigger drive would be limited to smallest  (assuming the controllers like each other enough that it actually works at all). 

Oh I know how it works, I'm just explaining the amount of drives I have @Mr.Meerkat

And yes it won't be the full size but better then nothing