Only one thing... Shakugan no Shana and Hayate the Combat Butler are diferent from Zero no Tsukaima.
The guys that own Zero no Tsukaima is Media Factory.
Knowing Media Factory "M.O."... there's a high probability for it to never be finished as anime...
Well... until the light novel ends (or the manga)... I don't see them making a 4th season... Unless they do the same as Maria+Holic... They made a anime (12 episodes) no real ending was ever given... and the manga is still going... and they don't plan on making more anime for it...
So... Lets wait and see...