This isnt really a trilogy per se since its not one story arc (hell, the first movie didnt even have a real story arc per se)..
the problem with trilogies that are set up that way is the story arc usually gets fucked. Almost allways the first movie is very self contained because they dont know if they will get to make the sequals or not.. so they write the script to tie up most of the loose ends nicely even if they dont solve everything (see the Matrix, had the sequals never been made, you would just assume Neo was in control, or Star Wars.. the death star was blown up.. and while Vader got away, had their never been sequals you still would have had some resolution)...
the problem comes with the next two movies.. you left everyone on a happy note at the end of the first, so you have to create some massive problem to be solved in the third film. This usually leaves audiences PISSED because Han is frozen in carbonite. Star Wars did an awesome job in the second movie by diminishing what had been accomplished in the first movie and showing the empire still firmly in control.. the matrix fucked up by having Neo way too powerfull the entire time, and waited until the last 10 minutes of reloaded to set up the conflict to be solved in the last movie.
Oh well, Im ranting.. but Im really looking forward to these movies.