This paper proposes a novel avoidance method called the Three-Dimensional Velocity Obstacle (3DVO) method. The method is designed for Unmanned Aerial Vehicle (UAV) applications, in particular to autonomously handle uncoordinated multiple encounters in an integrated airspace, by exploiting the limited space in a three-dimensional manner. The method is a three-dimensional extension of the Velocity Obstacle method that can reactively generate an avoidance maneuver by changing the vehicle velocity vector based on the encounter geometry. Adverse maneuvers of the obstacle are anticipated by introducing the concept of a buffer velocity set, which ensures that the ownship will diverge with sufficient space in case of sudden imminence. A three-dimensional resolution is generated by choosing the right plane for avoidance, in which the UAV conducts a pure turning maneuver. Implementation of the 3DVO method is tested in several simulations that demonstrate its capability to resolve various three-dimensional conflicts. A validation using Monte Carlo simulations is also conducted in stressful super-conflict scenarios, which results in zero collisions occurrences for the entire 25,000 samples. Nomenclature BV Buffer Velocity set CC Collision Cone set P φ Avoidance Plane at the angle φ RV i Obstacle Reachable Velocity set S pz Protected Zone VO P φ Velocity Obstacle section on Avoidance Plane-φ VO Velocity Obstacle set α vo Opening angle of the Velocity Obstacle Cone, [-] ∆t Time-step of the VO set generation, [s] δ P Dihedral angle of the avoidance-plane from the XY-plane, [-] ω a.cr Critical avoidance turning rate, [-/s] ω avo Avoidance turning rate, [-/s] φ P Angle of Avoidance Plane around X-axis ψ oi Azimuth angle of the obstacle-i from the ownship, [-] σ col Standard Distribution of Collision Probability, [-] θ oi Elevation angle of the obstacle-i from the ownship, [-] A vo Apex position of the Velocity Obstacle cone, [m] D vo Velocity Obstacle cone symmetric axis vector, [m] E φ vo Escaping point of a VO set, [m/s]