  We prove a necessary condition for the existence of the $A_p$-structure on
${\rm mod}~p$ spaces, and also derive a simple proof for the finiteness of the
number of ${\rm mod}~p$ $A_p$-spaces of given rank. As a direct application, we
compute a list of possible types of rank $3$ ${\rm mod}~3$ homotopy associative
$H$-spaces.
