I was asked this question, recently. My answer:
If you plan to home educate, I would not pick 3rd grade as a random start date. Children who attend school first and then are homeschooled often find the adjustment is more difficult. This is because these children quickly come to believe that the teacher in the school has the keys to the kingdom of knowledge and understanding. So, when you begin to explain long division, you will hear: “No, that’s not how Miss Magill does it. That is wrong, mom.”
Young children are not mature enough to understand that there are a dozen ways to get the job done and they are all legitimate. The goal is understanding. You can spare yourself this angst by not waiting to test the homeschool waters.
In school there is soooo much “process” that it becomes about the process rather than the content. You want to skip over this nonsense.
The families I counsel whose kids were in school for a while and were then removed from school, often have a difficult if not contentious first year, until they have the Aha(!) moment, when they realize that there are different ways to acquire understanding and mastery.
I think it is better to give home education a shot right away. Do not commit to do it for 5 or 10 years. Just do it for a year and do your best. Have fun. If it works, then do it for another year, etc., etc.