Due to Lanthanoid contraction their is greater decrease in ionic radii of elements in lanthanide series because of poor shielding of nuclear charge by $4\mathrm{f}$ electrons, hence the order will be $\mathrm{Y}{\mathrm{b}}^{+3}<\mathrm{P}{\mathrm{m}}^{3+}<\mathrm{C}{\mathrm{e}}^{+3}<\mathrm{L}{\mathrm{a}}^{3+}$