A light source device, an illumination device, and a projector which are capable of thinning a bundle of light beams while being low in a focal magnification ratio, and are small in size are to be provided. The light source device includes a first light source unit adapted to emit a first light beam, a second light source unit adapted to emit a bundle of light beams including a second light beam and a third light beam, a reduction optical system adapted to reduce the bundle of light beams, and a combining optical system adapted to combine the first light beam and the reduced bundle of light beams with each other. The combining optical system is provided with a light transmitting area and a light reflecting area.