How to know I'm ready to have a third child
As cliche as it sounds, you'll just know. You will either want another child or you won't, and your significant other will either agree or they won't. And sometimes just when we think we can't or don't want another child, it happens anyways. Then the decision is made for us. :)
Personally, as much work as it was to have children close together when they were little, it is so nice that they are each other's playmates. I love the closeness they have now.