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Cavity structures, like weapons bays and landing gear wells on aircraft, suffer from severe oscillations under high speed flow conditions. These oscillations are associated with intense surface pressure/velocity fluctuations inside the cavity which can radiate strong acoustic waves and cause structural damage. The physics of cavity flows have been studied for several decades with much of the effort put towards flow controls to reduce these oscillations. Geometric modifications of the cavity structure are usually only effective for suppressing the oscillations within the designed flow conditions. Therefore, active flow control is more attractive for a wider application range. Previous research have proven that mass/momentum injection at the cavity leading edge can effectively suppress the pressure/velocity fluctuations. Due to the limited control authorities of current actuators, a steady actuation which introduces three-dimensional disturbances is studied to reduce the energy requirements of the actuator and improve the suppression of the oscillations over a wide range of free-stream Mach numbers. Surface fluctuating pressure measurements are acquired to determine the control performances of a number of 3-D actuation configurations. Flow fields, including velocity fields and density gradient fields, are measured to reveal the flow features with and without the flow control. Mathematical methods, including modal decomposition analysis, are further applied to study the dynamics of the flow field. All of these analyses together elucidate the effective 3-D actuation mechanism in the cavity flow control. The suppression of pressure fluctuations are obtained in both full-span and finite-span cavities. The successful flow control is found to be the redistribution of the energy in the shear layer by the counter-rotating-vortex pairs, which are introduced by the 3-D actuation in the cross-flow. In addition, a design guide for the actuator geometry is given based on the observations.