Hello acslacker
All you need to do is choose RAID 5, select the 3 disks, and accept. It will automatically use all available space.
With your 3x1TB HDDs the maximum available space will be approximately 2TB. Some will disappear in the conversion to binary. This is why it is listing as 930GB instead of 1TB. In RAID 5 1 HDD is used for parity, so you will immediately lose 1TB. That space will not be displayed in the usable space. The 1.8TB that it is automatically listing sounds correct.
Thanks