  In three-dimensional critical percolation we study numerically the number of
clusters, $N_{\Gamma}$, which intersect a given subset of bonds, $\Gamma$. If
$\Gamma$ represents the interface between a subsystem and the environment, then
$N_{\Gamma}$ is related to the entanglement entropy of the critical diluted
quantum Ising model. Due to corners in $\Gamma$ there are singular corrections
to $N_{\Gamma}$, which scale as $b_{\Gamma} \ln L_{\Gamma}$, $L_{\Gamma}$ being
the linear size of $\Gamma$ and the prefactor, $b_{\Gamma}$, is found to be
universal. This result indicates that logarithmic finite-size corrections exist
in the free-energy of three-dimensional critical systems.
