The syntheses, structures, and characterization of three new mixed metal selenites, Al2(Se2O5)3, Ga2(Se2O5)3, and In2(Se2O5)3 are reported. Al2(Se2O5)3 and Ga2(Se2O5)3 are isostructural to Cr2(Se2O5)3 and crystallize in the monoclinic space group P21/n (No. 14) with a = 10.825(6) Å, b = 11.214(3) Å, c = 11.567(6) Å, and β = 94.92(3)° for Al2(Se2O5)3 and a = 10.901(6) Å, b = 11.367(3) Å, c = 11.682(3) Å, and β = 95.07(3)° for Ga2(Se2O5)3. The third material, In2(Se2O5)3, crystallizes in the non-centrosymmetric space group Pc (No. 7) with a = 11.1463(5) Å, b = 13.3210(6) Å, c = 9.7149(4) Å, β = 105.478(1)°, and Z = 4. Nonlinear optical measurements on In2(Se2O5)3 indicate the material has second harmonic generating properties, with an efficiency of approximately 10 times SiO2. Infrared and thermogravimetric analyses for all three compounds are also presented.