3. On the sides $AB, BC, CD, DA$ of the square $ABCD$, points $E, F, G, H$ are taken arbitrarily. Among the quadrilaterals $EFGH$ obtained, the number of squares is ( ).
(A) 1
(B) 2
(C) 4
(D) infinitely many
3.D.

As shown in Figure 5, as long as $A E=B F=C G=D H$, quadrilateral $E F G H$ is a square.
Figure 5
Figure 6