A new class of resonant exchange of power among three co-propagating modes in dielectric optical waveguides exhibiting Kerr-like nonlinearity is reported. Unlike the coupling of modes travelling in different, spatially separated channels as in the directional coupler (Jensen, IEEE J. Quantum Electron. QE-18 (1982) 1580; Chen, Snyder and Mitchell, Electron. Lett. 26 (1990) 77; Schmidt-Hattenberger, Trutschel and Lederer, Optics Lett. 16 (1991) 294) this paper points out the possibility of obtaining an efficient coupling of modes travelling in one single channel. The key point here is the imposition of a geometrical resonance between modes, to improve the coupling process. Due to this resonance condition, which is related with the wavelength through the propagation constants, the power exchange process is very sensitive to the wavelength.