New manganese arsenides CsMn4As3, RbMn4As3, and KMn4As3 were synthesized by solid-state reaction. They consist of edge-sharing MnAs4 tetrahedra, which are a building block similar to those of Fe-based superconductors. CsMn4As3 and RbMn4As3 adopt the KCu4S3-type structure (tetragonal P4/ mmm space group, No. 123) with a Mn4As3 double layer, while KMn4As3 has the CaFe4As3-type structure (orthorhombic Pnma space group, No. 62) with a Mn4As3 tunnel framework. The structural change from CsMn4As3 and RbMn4As3 to KMn4As3 as well as the structural trend of the other ternary A-Mn-As (A = alkali metal) and AE-Mn-As (AE = alkaline-earth metal) compounds is understood as a consequence of reduction of the coordination number around the A and AE sites owing to the decrease of the ionic radius from Cs+ to Mg2+. Electrical resistivity measurements confirm that the three new phases are Mott insulators with band gaps of 0.52 (CsMn4As3), 0.43 (RbMn4As3), and 0.31 eV (KMn4As3). Magnetic and heat capacity measurements revealed that CsMn4As3 and RbMn4As3 are antiferromagnets without apparent phase transitions below 400 K, which is similar to the magnetism of LaMnAsO and BaMn2As2, while the existence of the ferromagnetic component was indicated in KMn4As3 with a magnetic transition at 179 K.