What you're seeing is economic darwinism. Why is there a Starbux every three blocks? Because they're all making money. Given the rents they're paying, they have to be. The chain stores are more effective and efficient, and in lots of quanifiable ways, serve their customers better. So the customers vote with their feet and shop there.
And, why are they every three blocks? How far are you willing to walk for a cup of coffee (or to fill a prescription, or to deposit a check, etc.). NYC in general, and Manhattan in particular is a pedestrian town. every 3-6 blocks is a separate market. So the stores figure out what interval they need to be at, and the locate at that interval.