Integrated optics technologies are an attractive alternative to classical bulk optics for the beam combination function of an interferometer. We propose a new integrated optics combiner for three apertures giving access to the closure phase on each output. It uses a multimode interference combination scheme realized by ion exchange on a glass substrate. This paper describes the theoretical behaviour of the beam combiner and its design constraints. Its interferometric behaviour is simulated and first experimental results using for the first time, as far as we know, a Near field Scanning Optical Measurement (NSOM) technique are discussed.