Keep Movieclip Button in Down State Once Clicked

I have a navigation menu that needs to “highlight” the section you’re in. I have three buttons that I created as movieclips so they can have rollover/rollout animations. What I need to add on top of that, which I’m having trouble with, is that once the button is clicked, it stays in the “down” state until another button is clicked. Then it plays the rollout animation. I’ve looked at several threads for how to accomplish this, and tried my own workarounds but I’m not having any luck. Can anyone here point me in the right direction for this? I’ve attached an example of my project if that helps.

Also, if it changes anything, I’ll also need to add something that detects which section you’re in and automatically loads the corresponding button in the “down” state. (The homepage of this project has buttons that lead to each of the sections, and then this menu is loaded once you click one of those buttons… so depending on which button you select from the homepage, this menu will load with that button in the “down” state).

Thanks in advance!