  Ac susceptibility and static magnetization measurements were performed in the
nearly-optimally doped LaFeAsO$_{0.9}$F$_{0.1}$ and CeFeAsO$_{0.92}$F$_{0.08}$
superconductors, complementing earlier results on SmFeAsO$_{0.8}$F$_{0.2}$
[Phys. Rev. {\bf B 83}, 174514 (2011)]. The magnetic field -- temperature phase
diagram of the mixed superconducting state is drawn for the three materials,
displaying a sizeable reduction of the liquid phase upon increasing $T_{c}$ in
the range of applied fields ($H \leq 5$ T). This result indicates that
SmFeAsO$_{0.8}$F$_{0.2}$ is the most interesting compound among the
investigated ones in view of possible applications. The field-dependence of the
intra-grain depinning energy $U_{0}$ exhibits a common trend for all the
samples with a typical crossover field value (2500 Oe $\lesssim H_{cr} \lesssim
5000$ Oe) separating regions where single and collective depinning processes
are at work. Analysis of the data in terms of a simple two-fluid picture for
slightly anisotropic materials allows to estimate the zero-temperature
penetration depth $\lambda_{ab}(0)$ and the anisotropy parameter $\gamma$ for
the three materials. Finally, a sizeable suppression of the superfluid density
is deduced in a $s^{\pm}$ two-gap scenario.
