pressing ctrl, shift and pgUp/Down on cintiq 21

hi,
im using animate 2 academic version and cintiq 21ux.
and here is my question: i have mapped 3 of cintiq buttons for ctrl, shift and pgUp/Down and when i press all 3, it works as i press ctrl and pgUp/Down. so i dont have fuction “send to back/up” but i have “send backward/forward”. or doesnt cintiq realize 3 buttons at ones?