An optimizing method for the core arrangemnt of three-mode twelve-core fiber was proposed. After investigating the characteristics such as inter-core crosstalk, threshold value of bending radius and Relative Core Multiplicity Factor (RCMF), the optimized scheme for One-Ring Structure (ORS) and Square-Lattice Structure (SLS) were presented. For ORS, it is hard to find an appropriate core deployment to meet both requirements that crosstalk is not higher than -30 dB/100 km at the wavelength of 1 625 nm and the cable cutoff wavelength is not larger than 1 530 nm. For SLS, crosstalk can be lower than -30 dB/100 km after bending radius being larger than the threshold value, maximum cutoff wavelength is not larger than 1530 nm and RCMF can reach ~15 at the wavelength of 1625 nm, which indicates that SLS is a more suitable core arrangement than ORS for the three-mode twelve-core fiber.