In this communication, a wideband filtering antenna with folded structure is proposed for 5G n258 band millimeter-wave applications, where an air-filled metallic cavity machined by 3-D-printing technology is adopted as the basic structure for low-loss purpose. Based on multimode resonant theory, two cavity modes (TE110 and TE120) and one slot mode are excited simultaneously to attain the desired wideband performance. Then, electromagnetic (EM) couplings between cavity and slot modes are employed to bring two radiation nulls at two sides of the passband. Furthermore, by loading two additional shorting columns at the maximum ${E}$ -field intensity of TE220 mode, a third radiation null at higher stopband is introduced, giving rise to better out-of-band selectivity with wide suppression range at the higher band. Besides, with the help of 3-D-printing technology, the antenna cavity is deliberately folded, not only minimizing the antenna footprint but also improving the pattern asymmetry and the gain values at the broadside. Finally, the measurement is executed to experimentally verify the simulated performance of the proposed antenna, showing a wide bandwidth of 18.8% with a center frequency of 26.6 GHz, a peak gain of 8.8 dBi, and three radiation nulls at 23.2, 30.8, and 40 GHz.