3. If a straight line $l$ passes through three different points $A(a, b)$, $B(b, a)$, and $C(a-b, b-a)$, then the line $l$ passes through ( ) quadrants.
(A) II, IV
(B) I, III
(C) II, III, IV
(D) I, III, IV
3. (A).

The equation of the line passing through $A(a, b)$ and $B(b, a)$ is $y=-x+a+b$.
Substituting the coordinates of point $C(a-b, b-a)$ into the above equation yields $a+b=0$.

Therefore, the equation of line $l$ is $y=-x$.