The closest thing is setting all_users_control_menu = "true". However, this would obviously mean that all users would be able to control the menu and not just player three. However, you could unbind the controller mappings for all RetroArch functions and map them solely as keyboard keys. Then players one and two wouldn't have access from the controllers, but anyone using a keyboard would.
@maxxtc Scraped info is kept in /opt/retropie/config/all/emulationstation/gamelists (also available via file shares at \\retropie\configs\all\emulationstation\gamelists, each system has its own gamelist.xml that stores this info.
If you wish to remove this info, then exit Emulationstation and delete the gamelist.xml files with the wrong info. Starting Emulationstation again will re-create those files from scratch, but without any of the info you scraped.
@retroprogrammer No problem. The switch issue on Raspberry Pi is a need for everyone, not just RetroPie users. So, you can google the idea and find all kinds of examples of people using a simple momentary switch to short a GPIO pin to ground with examples of their custom scripts to monitor for that event. In most cases, examples in include a shutdown command, and that would be enough to do a soft shutdown or reboot. However, it is this command that you could replace with something more retropie-specific found in the threads here.