Since this is a NAS, keep in mind Network speeds. A three drive HDD RAID-Z1 is going to have pretty fast read speeds. It won't saturate gigabit Ethernet but it will be fast. SSDs have will be able to saturate the network but you'll have a fraction of the storage.
For me I'd rather have the extra space and just wait a few seconds longer to transfer stuff. Since you're using ZFS you might consider a small SSD (no more than 1TB) as a ZIL. It'll speed up writes and help plug the write hole on the NAS.
I've got a 4-disk RAID-Z1 and read speeds will come pretty close to saturating my gigabit Ethernet. My write speeds are a pretty consistent 60MB/s over the network. I'd really only go with SSDs if the NAS was an intermediary box that absolutely needed fast write speeds but then itself persisted to a larger but slower HDD box.
In general though I'm after storage primarily and speed second.