  We study the newly reported hidden-charm pentaquark candidates $P_c(4312)$,
$P_c(4440)$ and $P_c(4457)$ from the LHCb Collaboration, in the framework of
the effective-range expansion and resonance compositeness relations. The
scattering lengths and effective ranges from the $S$-wave $\Sigma_c\bar{D}$ and
$\Sigma_c\bar{D}^*$ scattering are calculated by using the experimental results
of the masses and widths of the $P_c(4312)$, $P_c(4440)$ and $P_c(4457)$. Then
we calculate the couplings between the $J/\psi p,\,\Sigma_c\bar{D}$ channels
and the pentaquark candidate $P_c(4312)$, with which we further estimate the
probabilities of finding the $J/\psi p$ and $\Sigma_c\bar{D}$ components inside
$P_c(4312)$. The partial decay widths and compositeness coefficients are
calculated for the $P_c(4440)$ and $P_c(4457)$ states by including the $J/\psi
p$ and $\Sigma_c\bar{D}^*$ channels. Similar studies are also carried out for
the three $P_c$ states by including the $\Lambda_c\bar{D}^{*}$ and
$\Sigma_c\bar{D}^{(*)}$ channels.
