Space-division multiplexing based on few-mode multi-core fiber (FM-MCF) technology is expected to break the Shannon limit of a single-mode fiber. However, an FM-MCF is compact, and it is difficult to couple the beam to each fiber core. 3D waveguide devices have the advantages of low insertion loss and low cross talk in separating various spatial paths of multi-core fibers. Designing a 3D waveguide device for an FM-MCF requires considering not only higher-order modes transmission, but also waveguide bending. We propose and demonstrate a 3D waveguide device fabricated by femtosecond laser direct writing for various spatial path separations in an FM-MCF. The 3D waveguide device couples the LP01 and LP11a modes to the FM-MCF with an insertion loss below 3 dB and cross talk between waveguides below $-36\mathrm{dB}$. To test the performance of the 3D waveguide device, we demonstrate four-channel multiplexing communication with two LP modes and two cores in a 1-km few-mode seven-core fiber. The bit error rate curves show that the different degrees of bending of the waveguides result in a difference of approximately 1 dB in the power penalty. Femtosecond laser direct writing fabrication enables 3D waveguide devices to support high-order LP modes transmission and further improves FM-MCF communication.