The big thing to keep in mind is - if you use the WPPSI-IV, I understood that for under 4, there are fewer sub-tests than over 4, and thus why most people find it more inconsistent for kids under 4 years old (granted, even under 7/8 years old, results can be inconsistent). I am not sure how different the test is when the child is over 4 versus a younger child though. I don't know about SB-V since it seems like more people use WPPSI for preschool age children and DS took the WPPSI-IV.
We did not test at 3 because of concerns about unreliability of the results either way. We tested at 4 using the WPPSI and that is a good age to do so, in my opinion.
ETA: Since you are in CA and hoping for early K entry, I recommend that you look up the K curriculum (available if you google it on the state's website). You can then compare it with what your DS already does and get an idea if early K entry is going to be sufficient for your DS or not.
