We propose a three-dimensional waveguide structure for the realization of mode-selective switches, where the high-order spatial modes of a few-mode waveguide can be switched to various single-mode waveguides with thermo-optically controlled vertical asymmetrical directional couplers. To demonstrate the idea, we design and fabricate two specific devices with polymer material for a waveguide that supports three spatial modes. The two devices differ in the application of the thermo-optic effect to deactivate or activate the directional couplers. For one device, the switching powers measured at the wavelength 1550 nm are lower than 20.6 mW and the switching times are shorter than 4.4 ms. The extinction ratios are higher than 15.6 dB across the C-band measured at fixed switching powers. For the other device, the switching powers measured at 1550 nm are lower than 22.6 mW, the switching times are shorter than 2.9 ms, and the extinction ratios are higher than 14.1 dB in the C-band. The performances of the devices are weakly sensitive to the polarization state of light. Our proposed mode-selective switches, which have a scalable configuration and require low power consumption, could be developed into various active mode-controlling devices for applications in reconfigurable mode-division- multiplexing networks.