Four types of three-waveguide optical switches are analyzed by measns of the coupled mode theory. Assuming that three proximate waveguides of an optical switch are identical and equally spaced, we examine the switching behavior for both cases where an incident beam is launched into 1) the outer waveguide, 2) the center waveguide. For each beam-launching case, switching voltage is applied so that three waveguides refractive indices be detuned 1) symmetric, 2) anti-symmetric. It is concluded that only three types of optical switches out of four an perform the switching operation. We derive the switching conditions for ach of three possible types and draw the corresponding switching diagrams.