you would hear all three as they are mixed within that folder. It would depend on having any Mutes or Solos active also.
You're watching a Kenny Video I assume? there are more than one video available concerning Folders, Groups, and VCA's, and all are easy to absorb. Though in my personal adventures I have watched some of the Vid's more than a couple times to really catch all the subtle details..
Also page 93 of the
user manual has three pic's that help in visualizing a folder and how it works compared to other options