This is probably related to the puzzle of how many children you need in a class to be fairly certain that two of them will share the same birthday. The answer is surprisingly small less than 30 if I recall. The trick is that it is not the odds of 3 incidents in Grey close but the odds of 3 incidents in any one street. Hope that helps or there's a statistician soon along to explain better. (what are the odds of a trained statistician reading this post?)