One of the reasons this seems more natural is that it is what one would observe in the real world-- the random nature of reality dictates that three characters would be in different stages of walking.
But there's a more compelling principle at work here, what I call fictitious action. The reader seeing these three characters has a tendency to feel closure between them (closure as McCloud defines it in UC). Even as the reader recognizes that they are three different characters, the reader perceives them as an illustration of the act of walking, showing different stages of the walking process-- right foot forward, mid-step, left foot forward.
To realize this effect, it isn't particularly important to show the characters in precisely-ordered stages of walking; right-foot/left-foot/mid-step works as well as right-foot/mid-step/left-foot. What's important is to show CHANGE taking place. What makes in-step walking appear static is that no change is being shown.
Fictitious action is any appearance of action in a comic that results from a closure between two unlike subjects. This action is not perceived as being literal, but it enhances the liveliness and realism of the comic by making it seem as if things are in motion.
By 'unlike subjects' I mean subjects that are either completely distinct from one another, like the three people walking in tandem; or the same subject in two pictures that are remote from each other in time and place.
Unlike subjects can be occupying the same panel, as the three walkers in tandem are. Or they can be in adjacent panels. By adjacent, I don't necessarily mean sequentially next to each other; fictitious action can hopscotch down the rows of panels, or from one page to the next.
As a rule, every new character shown walking in a sequence should be in a different step from their predecessors. If three different characters are in three panels in Paris, Rome, and France respectively, they should still be shown left-foot/right-foot/mid-step.
Of course, this is a rather subtle thing, and violations of the principle aren't very noticable. But it's a factor in making the story dynamic and lively.
I've used walking as my primary example so far, but any type of action can be subject to this type of fictitious closure. Three people eating in a restaurant, for instance-- one is poking a fork into the broccoli, the other has a spoon to her lips, the third is taking a sip of coffee. Or swimmers-- one is diving, another is doing a dog-paddle, while a third is climbing out of the pool.
Fictitious action fleshes out the motion and activity in a comic, but it can also be exploited to suggest ideas. Two characters who are sharing an action will seem to be in communion with one another, even if they are far apart. A man opening a closet door followed by a woman closing a book can have a romantic overtone.
Fictitious action can help carry the flow of the narrative. The completion of a fictitious action over a series of scenes can suggest the completion of a phase of the story. It can suggest a momentum of forward motion, or a series of blocked actions.
Rarely brought to the surface, fictitious action is nevertheless a vital part of comics vocabulary. Comics theorists, fans, and artists should learn to recognize it and appreciate the important role it plays.