We propose three-dimensional (3D) polymer directional couplers operating at 1550 nm for on-board optical interconnects. The proposed directional coupler can realize free-position light coupling and splitting in three dimension and can be cascaded to serve as an effective and scalable device for multi-layer and multi-channel interconnects. The 3D directional coupler was fabricated with commercially available UV-curable epoxies by combining the use of a needle-type liquid micro-dispenser and a 3-axis robot stage. The main parameters of the 3D directional coupler such as the interaction length, coupling ratio, coupling position, and cascade numbers can be easily adjusted. In the experiment, single-mode two-dimensional (2D) polymer directional couplers in both horizontal and vertical directions operating at 1550 nm with different coupling ratios (CRs) were demonstrated at first. The experimental results agree with the simulated ones. A 3D directional coupler with coupling ratios of 58:23:19 at 1550 nm was then successfully fabricated using the mosquito method for the first time to the best of our knowledge. The CR varies with the wavelength as expected while the excess loss remains almost the same within C-band. The results imply that the proposed method may be applicable in the fabrication of functional devices in three dimension for high-density on-board optical interconnects with further improvement on fabrication process.