Yes, this build does seem a bit silly to have 3 phantom dancers, but notice you have 90 crit chance and very high attack speed. another great thing about this build is, because Hecarim gets more AD for movement speed, the Phantom Dancers add even MORE AD in addition to the base AD they add. Overall I've found this build and slight variations of it to be VERY effective. Getting red buff or replacing a phantom dancer with a Frozen Mallet also helps greatly, as enemies will not be able to escape you.