Rotors with blades, as in wind turbines, are prone to vibrations due to the flexibility of the blades and the support. In the present paper a theory is developed for active control of a combined set of vibration modes in three-bladed rotors. The control system consists of identical collocated actuator-sensor pairs on each of the blades, and targets aset of three modes constituting a collective mode with identical motion of all the blades, and two independent whirling modes, in which a relative motion pattern moves forward or backward over the rotor. The natural frequency of the collective mode is typically lower than the frequency of the whirling modes due to support flexibility. The control signals from the blades are combined into a mean signal, addressing the collective mode, and three components from which the mean signal has be subtracted, addressing the pair of whirling modes. The response of the actuators is tuned to provide resonant damping of the collective mode and the whirling modes by using the separate resonance characteristics of the collective and the whirling modes. In the calibration of the control parameters it is important to account for the added flexibility of the structure due to influence of other nonresonant modes. The efficiency of the method isdemonstrated byapplication to a rotor with 42 m blades, where the sensor/actuator system is implemented in the form of an axial extensible strut near the root of each blade. The load is provided by a simple but fully threedimensional correlated wind velocity field. It is shown by numerical simulations that the active damping system can provide a significant reduction in the response amplitude of the targeted modes, while applying control moments to the blades that are about 1 order of magnitude smaller than the moments from the external load. Copyright © 2011.