static int touchpad_buttons[3][3] = {
    {BUTTON_TOPLEFT, BUTTON_TOPMIDDLE, BUTTON_TOPRIGHT},
    {BUTTON_MIDLEFT, BUTTON_CENTER, BUTTON_MIDRIGHT},
    {BUTTON_BOTTOMLEFT, BUTTON_BOTTOMMIDDLE, BUTTON_BOTTOMRIGHT},
};

....
y is current y pixel, x is x pixel

r_button |= touchpad_buttons[y/(LCD_HEIGHT/3)][x/(LCD_WIDTH/3)];