If I was on my parent's health insurance until August 2023, then had my own insurance, what box do I check for Line 3A6?

First, go back to the HSA interview and when you are asked "Was [name] covered by a High Deductible Health Plan (HDHP) in 2023?" answer, "No, I did not have coverage." This is the correct answer for someone who could be claimed as a dependent, whether or not you also had HDHP coverage.

Then in the Review, when you are asked the question about Line 1 (I think this will happen now on Line 1, not 3) answer "Self". 

This is OK because your first answer above caused the calculations to be correct, and the second answer will get you past the check in the Review.

Come back if that doesn't work for you.