ATechnically the movie is nothing but a 3-episode series with extensions to combine the plot of the episodes. That's exactly how it was aired in May 2006 in the US. The three episodes are the last episodes if season 4 (4x28, 4x29, 4x30). BUT: as usual for newer episodes, the footage had to be censored for the US TV. On the one hand a lot scenes are missing to reach the common running time of the episodes, on the other hand lots of modification were made because it was too rough for TV. With a running time of 88 min, the observing audience notices that it's much longer than just 3 episodes (22 min each including opening and final credits). That is because of the additional frame story. Before and after the feature film, we see the Griffins at the movie premiere and the after-show. That added footage runs approx. 16 min. Nevertheless there are also more than 10 min of additional footage in the actual episodes, so that the fan shouldn't have anything to complain about.
While Episode 2 only contains some minor modifications, it's especially Episode 3 that contains many differences. Episode 1 is somewhere in between. A detailed comparison between both versions with pictures can be found here.