We propose and demonstrate a novel method that uses only three sets of B-scans to accurately determine both the direction and the speed of a transversal flow using speckle decorrelation optical coherence tomography. Our tri-scan method has the advantages of high measurement speed, high spatial resolution, and insensitivity to the flow speed. By introducing error maps, we show that the flow angle inaccuracy can be minimized by choosing the measurement result with a lesser error between results obtained from the x-and y-scans. Finally, we demonstrate that the flow angle measurement accuracy can be further improved for the high-speed flows by increasing the speed of the x-and y-scans. (C) 2017 Optical Society of America