You probably could but i doubt it will give much noticeable gains, i personally wouldn't simply because the amount of power usage, heat output and case congestion that would be used running 3 cards in a system. I mean does he even have enough Power connectors to run this many cards?
As far as i know some games heavy on PhysX do show some gains from a dedicated PhysX card, whilst overs get no gain whatsoever, hence my opinion above.