Technically you can do mirror, raidz, or raidz2, not just mirror or raidz2.
There is absolutely no reason to use raidz2 on a 3-disk set. You will get effectively nearly an equivalent amount of usable space out of those 3 disks as if you had mirrored them, but with significant additional complexity for I/O and a lower read speed. ZFS mirrors can round-robin read access across all spindles within the vdev, whilst raidz cannot.
So really the only question here is raidz (raidz1) or mirror. And then you're back into the more familiar world of data resiliency versus capacity. A 3-way mirror is significantly more resilient than a 3-disk raidz vdev, but a 3-disk raidz vdev has about twice the usable space as the 3-way mirror.