3. The vertex of the parabola $y=a x^{2}+b x+c$ is $(4,-11)$, and the x-coordinates of its two intersections with the x-axis are one positive and one negative. Then, among $a$, $b$, and $c$, the positive number(s) is/are ( ).
(A) only $a$
(B) only $b$
(C) only $c$
(D) $a$ and $b$
3.A.

Given the vertex is $(4,-11)$, and the parabola intersects the $x$-axis at two points, we know $a > 0$. Let the $x$-coordinates of the two intersection points of the parabola with the $x$-axis be $x_{1}$ and $x_{2}$, i.e., $x_{1}$ and $x_{2}$ are the two roots of the equation $a x^{2}+b x+c=0$.

From the given $x_{1} x_{2}0$, it follows that $b<0$.