Rubens places the three around a Roman-style pillar to add a touch of elegance and class to this charming artwork. With many elements of the scene left in relatively little detail, this is clearly a study drawing for an element of a later painting. Perhaps these women were to be a background feature to a larger artwork.
This style of portraying mythological women can be seen across several other art movements, most frequently within the Pre-Raphaelites of several centuries later. They used traditional content within a charming style that captured these female figures within the British countryside and that movement continues to be particularly popular today, despite the lack of international academic backing. You will see paintings that make use of these female forms in a similar way to this Rubens drawing by the likes of John William Waterhouse, John Everett Millais and William Holman Hunt.
Peter Paul Rubens is famous for depicting women in a certain way, with curves and an honest, curvaceous frame that he felt was real and beautiful. In that sense, this drawing is a little out of sync, with these ladies being particularly slim and elegant, more in line with perhaps the work of William Adolphe Bouguereau.