If you want 3TB of usable storage, you'll need more (or larger) hard drives. You shouldn't use more than 80% of the storage space, as FreeNAS will start to lose performance once the volume gets too full. You should also consider that your storage space needs will certainly grow with time, so you don't want to start out already at 80% full. Also keep in mind the difference between TB and TiB.
250 GB for Time Machine seems quite small to me, as it is an incremental backup system.
You can calculate the usable storage space provided by various numbers of drives, drive sizes and RAIDZ levels, using the
nice web app created by
Bidule0hm - instructions are
here.