Page 72, article 8, B says, "The designated spot shall be 3' wide, with no depth limitation"
Also, article 9, B says, "No opponent of the thrower-in shall have any part of his person over the inside plane of the boundary line"
I think that reference to "3 feet" is referring to the width of the space that an in-bounder has to move. Hence the additional reference to there being no depth limitation (except in this case with the scorer's table and video board creating a depth limitation).