The three-component penetrable-sphere model is used to study the adsorption of a third component, b, at the surface of two fluid phases, α which is rich in component a, and γ which is rich in component c. The system has a tricritical point at a reduced pressure of πtc = 15/4, in a mean-field approximation. At and below this pressure the adsorption of b, Γb, first rises as the activity of b, ζb, increases and then falls rapidly as the tricritical or ordinary critical point of the α-γ system is reached. Above πtc the adsorption diverges to an infinite value as ζb approaches ζsat b, the value at which component b saturates both the α and γ phases, and so first forms a new bulk phase, β, which wets the α-γ interface. It is shown that Γb diverges as -ln(ζsat b - ζb), a divergence that had been derived previously by Widom on the basis of a phenomenologically constructed free energy.