Those types of accidents happen from time to time. I think that some of them happen when their feet are in the box but the butt hangs over the edge. If it was dirty and they wanted to deposit in a clean spot, maybe the only clean spot was over the edge.
But anyway, I am glad that you got the 3rd box. You won't regret it. Even if they had no accidents, as they age they may need to have that extra place to go potty. And another thing I always think about: If one had diarrhea and needs to go FAST, or worse, if 2 have diarrhea.........well, you get the idea.
Hope there is no medical problem.