The authors report the fabrication of high-silica, low-loss three-waveguide couplers (3WC) on Si by a combination of flame hydrolysis deposition (FHD), photolithographic patterning, and reactive ion etching. The device has been designed using the first-order perturbation theory, in order to achieve single-mode operation and to predict the beating length. Simulation of the device has been performed using the beam propagation method. SiO2-P2O5 glass has been employed as core material. P2O5 has been used to increase the refractive index of silica, as well as to decrease its sintering point. The sintering process has been optimized in order to achieve completely sintered, bubble free, low-loss films, preventing the evaporation of the P2O5 as well as the warping of the substrate. The devices have been characterized in terms of insertion loss, power splitting ratio and spectral response. 3WCs with multiple number of beating lengths have also been considered in order to assess the modulation of their spectral response with increasing interaction length.