As you are probably aware, RAIDZ1 means loosing all your data if two disks fail simultaneously. This is not a highly likely scenario, but it is not as uncommon as it sounds. Resilvering after a first disk failure puts a significant strain on the remaining disks which increases the likelihood of a second failure. Moreover, if you buy and start using all disks at the same time, they are more likely to go roughly at the same time. If they were produced roughly at the same time, they are also more likely to be affected by similar production errors that could make them fail simultaneously. On the positive side, the likelihood of a two disk failure is smaller in a three disk array than in one with eight disks, say.
If your data is important to you, the obvious solution would be adding another disk and going for RAIDZ2. If price is a concern, consider going for 4x3TB in RAIDZ2 which will give you 6TB of usable space at roughly the same cost as 3x4TB. If you want to stay with three disks, a three disk mirror will deliver the same redundancy as RAIDZ2, but significantly better performance.
A three disk RAIDZ1 with 4TB disks is not a completely bad decision though. It all depends on the value of your data. If you keep multiple frequent backups and can afford to lose/recreate all data created between backups; or if all the data can be recreated at an acceptable cost/effort; the setup should be fine.
With regard to backups, this is extra important as you appear to be going for consumer grade hardware. In principle, consumer grade hardware should be fine, but it does increase the risk of a complete pool failure somewhat (which is not so fine). Hence, while you should always keep backups, with this setup it becomes somewhat more likely that you will have to use them.
Finally, I would try to find another mainboard. The ASRock Z77 pro4 seems to be using an on-board ASMedia ASM1061 HBA for two of the four SATA ports. Older posts indicate that this HBA did previously not work with FreeNAS, but this may have been fixed. Another concern might be whether this HBA implements the SATA flush cash commands correctly (i.e. FLUSH CASH and FUA). Not all budget HBA’s and USB-to-SATA adapters seems to do this. Unfortunately, this is one of few hard requirements for ZFS to maintain a consistent on disk format and lack of it could potentially cause pool corruption in case of a power loss.
Ideally, you should be looking at a server grade mainboard with ECC memory. However, I understand that cost may be a concern. If you decide to go for consumer grade hardware, try to get a mainboard that supports all your disks on the CPU/PCH SATA controller or a LSI HBA.
