Classical formal portrait lighting is a high and wide main light (meaning up to 45 degrees higher and 45 degrees more to the side) of the subject's face. It must be a large closed light to be soft (umbrella or softbox).
The fill light is approximately 1 EV less bright (ratio of illumination in the subject) and frontal, which means that it is as close as possible to the lens axis (to fill in the exact shadows that the lens sees without creating another set of shades). A flash in the shoe can do that. Larger lights (umbrellas) are, of course, softer, but softness is not a problem for a flat front light that does not make shadows (on the face … if it is near the lens axis, then any shadow behind it must hide behind the subject).
The third light may be behind the subject at the bottom, or it may be a light of hair.
My site has a page that could be clarified, at https://www.scantips.com/lights/setup/