Solution: (1) The area of overlapping rectangle is 6 ÷ 2 = 3.

So the answer is: 3;

(2) As shown:

(3) If the abscissa of point P is not greater than 3 and the ordinate is equal to 3, then there is

P 1( 1，3)； P2(2，3)； P3(3.3)。