Even if it was pitched as a trilogy, if the first one tanked, they could still esily pull the plug on the next two. Why sink millions of dollars into two more films if they aren't going to make them some cash. Sequels generally make less money then their orginal.
Case in point, The Chronciles of Riddick was pitched as a trilogy as well, but after spending like $125 million on the first film, it's highly unlikely we'll see the the other two. Especially since it only made about $115 million worldwide. (Of course, after Vin made a shit load of cash in that Disney movie a few months back, someone will probably get the idea to move ahead with the next film anyway.)