We study the thermal behaviour of a tri-layer multilayer, designed by inserting a third material, yttrium, into the previously studied Co/Mo2C system. The system is designed to work at near-normal incidence at the wavelength of 14.1 nm. The theoretical reflectivity of Co-based multilayer (Co/Mo2C/Y system) is improved up to 54% after the addition of yttrium. Two types of multilayers with different orders of yttrium layer are deposited: Co/Mo2C/Y and Co/Y/Mo2C. The samples are annealed up to 600 °C. The multilayers were characterized using hard x-ray and extreme ultraviolet reflectivity, nuclear magnetic resonance (NMR) spectroscopy and x-ray diffraction (XRD). The results show that the reflectivity of the Co/Mo2C/Y multilayer is 27.5% at near normal incidence around 14.6 nm for as-deposited sample, and then it decreases gradually after annealing up to 600 °C. A significant period compression is observed from 300 °C annealing and above. The Co/Y/Mo2C multilayer shows low reflectivity, less than 2.5%. NMR spectra reveal that the pure Co layers are completely mixed with other elements since there is no signal from ferromagnetic Co in the annealing samples of the Co/Mo2C/Y multilayer and all Co/Y/Mo2C samples. Based on the NMR and XRD results, we fit the EUV data for both multilayers with two different models in one period taking into account the formation of the interfacial compounds.