Forks can activate (and rename) a third button too for the advanced settings and controls specific to their forks.
Modify this to enable the third button:
_btnOTHER->setText(tr("STR_ENGINE_OTHER")); // rename in your fork
_btnOTHER->setGroup(&_owner);
_btnOTHER->onMousePress((ActionHandler)&OptionsAdvancedState::btnGroupPress, SDL_BUTTON_LEFT);
_btnOTHER->setVisible(false); // enable in your fork
Please put your own advanced settings and controls in here (into createAdvancedOptionsOTHER() and createControlsOTHER()):
/**
* Sets up the options by creating their OptionInfo metadata.
*/
void create()
{
if (!_info.empty()) { _info.clear(); }
////////////////////////////////////////////////////////////
// OXC - OpenXcom
////////////////////////////////////////////////////////////
createOptionsOXC();
createAdvancedOptionsOXC();
createControlsOXC();
////////////////////////////////////////////////////////////
// OXCE - OpenXcom Extended
////////////////////////////////////////////////////////////
createOptionsOXCE();
createAdvancedOptionsOXCE();
createControlsOXCE();
////////////////////////////////////////////////////////////
// OTHER - your fork name here
////////////////////////////////////////////////////////////
createOptionsOTHER();
createAdvancedOptionsOTHER();
createControlsOTHER();
}