The time-resolved three-dimensional velocity measurement of an underexpanded jet is performed using time-resolved microphone and non-time-resolved particle image velocimetry (PIV) measurements. Flapping screech jets at the nozzle pressure ratio (NPR) of 2.97 was measured and analyzed using the proposed approach. It was found that the dominant azimuthal mode at the present case is flapping mode. The screech tones are strongly associated with the first two proper orthogonal decomposition modes of the dominant azimuthal modes of the microphone data. The azimuthal characteristics of these modes are helical mode, and the rotation direction is opposite. These findings are qualitatively consistent with the previous results. The flapping structure rotates over a wide range of temporal scales, with random changes in direction of rotation.