A pixel array in a three-dimensional image sensor includes depth pixels and an ambient light cancellation (ALC) circuit. The depth pixels operate in response to photo control signals having different phases, and generate distance information of an object based on light reflected by the object. The ALC circuit removes an ambient light component from the reflected light, and is shared by the depth pixels. Each depth pixel includes a photoelectric conversion region, a floating diffusion region, a photo gate, and a drain gate. The photoelectric conversion region collects photo charges based on the reflected light. The floating diffusion region accumulates the photo charges. The photo gate is activated in response to one of the photo control signals. The photoelectric conversion region accumulates the photo charges when the photo gate is activated, and the photo charges in the photoelectric conversion region are released when the drain gate is activated.