The number of valence electrons is increasing in an integral fashion. The change in the valency is according to the tendency to give up or borrow electrons. Thus elements in the same period have consecutive atomic numbers and different valencies. If we see the atomic radii across the period, we will notice that the size decreases across the period. Now let us consider the metallic character of the elements in the third period.
Figure below shows the same.
They give up the electrons in the last shell very easily. They are shiny in colour and conduct electricity. Al has 3 electrons in its outermost shell and behaves like a metal. It has 4 electrons in its outermost shell. It thus needs to borrow four electrons or give up all its four electrons to form a stable shell.