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 $Sigma_cbar{D}$ and $Sigma_cbar{D}^*$ 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 $J/psi p,,Sigma_cbar{D}$ channels and the pentaquark candidate $P_c(4312)$, with which we further estimate the probabilities of finding the $J/psi p$ and $Sigma_cbar{D}$ 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 $Sigma_cbar{D}^*$ channels. Similar studies are also carried out for the three $P_c$ states by including the $Lambda_cbar{D}^{*}$ and $Sigma_cbar{D}^{(*)}$ channels.
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