We demonstrate the performance of broadband, low loss, and compact three-mode converter and (de)multiplexer utilizing an asymmetric trijunction splitter, multimode interferometer, and a phase shifter; all are designed using on subwavelength grating technology (SWG) and built-in a silicon-on-insulator platform for the first time to the best of our knowledge. SWG enables the flexibility of refractive index engineering for more operating bandwidth and compactness compared with conventional counterparts. The designed device is capable of converting and (de)multiplexing three different input modes TE₀, TE₁, and TE₂ into a fundamental TE₀ output mode. The three-dimensional finite-difference time-domain simulation results confirm that the designed device has a low loss of 1 dB and the crosstalk reaches −65  dB over a broad wavelength of 160 nm (1470 to 1630 nm) in the C-band range with an overall size of the designed device is 95  ×  4  μm².