Common Building Motifs in Ba\\r <sub/>2sub>\\r Fe\\r <sub/>3sub>\\r (PO\\r <sub/>4sub>\\r )\\r <sub/>4sub>\\r ·2H\\r <sub/>2sub>\\r O, BaFe\\r <sub/>3sub>\\r (PO\\r <sub/>4sub>\\r )\\r <sub/>3,sub>\\r and Na\\r <sub/>3sub>\\r Fe\\r <sub/>3sub>\\r (PO\\r <sub/>4sub>\\r )\\r <sub/>4sub>\\r : Labile Fe\\r 2+\\r /Fe\\r 3+\\r Ordering and Charge-Dependent Magnetism