Mode multiplexers are essential components in space/mode division multiplexing. A thermally expanded two-core-fiber-based mode multiplexer/demultiplexer design is proposed, which multiplexes the LP01 and LP11 modes of a two-mode fiber. The extinction ratio of the proposed mode multiplexer is higher than 30 dB over the entire C-band. We extend this design to a three-core-fiber structure that can multiplex/demultiplex both LP11a and LP11b spatial modes. The extinction ratio of the three-core device is found to be more than 23 dB. The same device can work as a LP11 filter and LP11 tap coupler easily by changing either the core expansion or the scan length. © 2019 Optical Society of America.