<p>In response to the future demand for efficient and simplified high-lift configuration in civil aircraft, this study numerically investigates flow control on a swept-back multi-element wing using synthetic jets. To elucidate the differences in control mechanisms between two-dimensional and three-dimensional flows, we focus on the crossflow effect on the flow control mechanism by comparing the results on straight and swept wing configurations. The results show that the crossflow effect plays a crucial role in flow control. The swept wing achieves a lift gain of 10.7% under low-frequency excitation (<i>F</i>⁺ = 1), with relatively uniform spanwise distribution. In contrast, the straight wing exhibits only a 2.3% gain, and its control effect remains localized near the orifice. This limitation can be mitigated through high-frequency excitation at <i>F</i>⁺ = 10, which improves spanwise control effectiveness. Further analysis of the vortex dynamics reveals the underlying mechanism. The background crossflow on the swept wing effectively stretches and tilts the jet-induced vortex structures, promoting a robust spanwise transport of momentum that enables global control. In contrast, vortex structures on the straight wing primarily convect downstream, lacking this inherent spanwise spreading mechanism. These findings offer direct design guidelines for the design of three-dimensional flow control strategies. For the straight wing, the actuators require closely spaced arrangements to achieve global lift enhancement; for the swept wing, a sparse actuator arrangement can enable efficient large-area flow control by utilizing the crossflow effect.</p>