In attempts to synthesize compds. contg. mixed lanthanide and transition metal oxide magnetic nanostructures, a new 3d-4f arsenate phase, Na3GdMn3O3(AsO4)3, was isolated using high-temp., molten-salt methods. The X-ray single-crystal structure anal. shows that Na3GdMn3O3(AsO4)3 crystallizes in a hexagonal space group with a = 11.091(2) Å, c = 5.954(1) Å, and V = 634.2(2) Å3; P63/m (No.176); Z = 2. The structure has been refined by full-matrix, least-squares methods to a final soln. with R1 = 0.0466, wR2 = 0.1318 and GOF = 1.101. This new three-dimensional framework consists of ferromagnetic one-dimensional [MnO4]∞ chains composed of edge-shared MnO6 units propagating along c. The GdO9 polyhedra connect three neighboring [MnO4]∞ chains via oxygen atoms. Na3LaMn3O3(AsO4)3 (1), and Na3SmMn3O3(AsO4)3 (2), the isostructural derivs. of Na3GdMn3O3(AsO4)3 (3), were synthesized to acquire addnl. insight into the 3d-4f magnetic couplings. The Na3LnMn3O3(AsO4)3 series reveals an increase in χT at low temps. upon substitution of smaller Ln3+ ions ranging from La3+ to Gd3+. The increase in χT is likely due to the magnetic nature and size of the lanthanide ions present. It was expected that the Mn-O-Mn ferromagnetic interactions would dominate as 3d-4f magnetic interactions are generally weak. The magnetic data shows that the paramagnetic Ln3+ ions in 2 (Sm) and 3 (Gd) do significantly enhance the bulk ferromagnetism compared to that of 1 (La), suggesting sizable contributions from the 4f magnetic ions.