  In a chromomagnetic model, we analyse the properties of the newly observed
$P_c(4457)^+$, $P_c(4440)^+$, and $P_c(4312)^+$ states. We estimate the masses
of the $(uud)_{8_c}(c\bar{c})_{8_c}$ and $(uds)_{8_c}(c\bar{c})_{8_c}$
pentaquark states by considering the isospin breaking effects. Their values are
determined by calculating mass distances from the $\Sigma_c^{++}D^-$ and
$\Xi_c^{\prime+}D^-$ thresholds, respectively. It is found that the isospin
breaking effects on the spectrum are small. From the uncertainty consideration
and the rearrangement decay properties in a simple model, we find that it is
possible to assign the $P_c(4457)^+$, $P_c(4440)^+$, and $P_c(4312)^+$ as
$J^P=3/2^-$, $1/2^-$, and $3/2^-$ pentaquark states, respectively. The
assignment in the molecule picture can be different, in particular for the
$P_c(4312)^+$. The information from open-charm channels, e.g. ${\cal
B}[P_c\to\Sigma_c^{++}D^-]/{\cal B}[P_c\to J/\psi p]$, will play an important
role in distinguishing the inner structures of the $P_c$ states. Discussions
and predictions based on the calculations are also given.
