3. Let the parabola $C: y^{2}=2 p x(p>0)$ have a focus at $F$, and its directrix intersects the $x$-axis at point $Q$. A line through point $F$ intersects the parabola $C$ at points $A$ and $B$, and $\angle Q B F=90^{\circ}$. Then $|A F|-|B F|=$ $\qquad$
3. 2 p.

As shown in Figure 4, let the inclination angle of line $AB$ be the acute angle $\theta$. In the right triangle $\triangle B Q F$, applying the definition of a parabola, we get
$$
\sin ^{2} \theta=\cos \theta, \quad |B F|=p \cos \theta .
$$

Using the focal chord length formula, we have
$$
\begin{array}{l}
|A F|+|B F|=\frac{2 p}{\sin ^{2} \theta} . \\
\text { Then }|A F|-|B F|=(|A F|+|B F|)-2|B F| \\
=\frac{2 p}{\sin ^{2} \theta}-2 p \cos \theta=2 p\left(\frac{1}{\cos \theta}-\cos \theta\right) \\
=2 p \cdot \frac{1-\cos ^{2} \theta}{\cos \theta}=2 p \cdot \frac{\sin ^{2} \theta}{\cos \theta}=2 p .
\end{array}
$$