Subverted: Bob, Jack and Larry become Jack in the film version, but along comes Larry...
Double Subverted: ...Larry In Name Only, that is. Original Larry's traits are still with new Jack.
Bob, Jack and Larry become Jack in the film version, but he has Dissociative Identity Disorder and Bob and Larry are separate personalities.
Or Jack looks like Bob on the left and Larry on the right, with a clear line of demarcation where they've literally been spliced together.
Or, further along that line, Jack is obviously a Combining Mecha, with Bob forming the legs, Larry forming the arms and shoulders, and Jack forming the torso of the combined form of Film Jack.
Zig Zagged: Elements of Bob and Larry are added to Jack in the film version. But there's also an actual Larry, who isn't In Name Only. But then he gets given some of Bob and Jack's characteristics.
Averted: Any character that doesn't make it to the film version is simply left out, and nobody inherits their traits.
There were too many characters in the source; merging similar characters into each other makes things easy to follow.
Combining similar characters means paying for fewer actors and so cuts down on the cost of making the adaptation.
The book was long enough to have Tim take three jobs, the movie is is not. So instead of picking one of the jobs, they combined them all into one.
Lampshaded: Tim tells Jack that he reminds him of two of his old bosses, Bob and Larry.
Defied: Bob, Larry and Jack, despite any similarities, make it into the film version.
Discussed: "Sometimes, when doing an adaptation, we just have to move the traits of one character to another".
Conversed: "Sometimes, when doing an adaptation, the people doing that adaptation just have to move the traits of one character to another".
Deconstructed: Film Jack was created by a demon from merging Bob, Jack and Larry together; memories of his components' lives begin to haunt him and he desires to split apart.
Reconstructed: Film Jack decides to live his life as the single entity he is.
Plotted A Good Waste: Bob, Larry, and Jack were pretty Flat Characters in the source material, but now the most interesting bits of each have been granted to Film Jack, making Film Jack a more interesting character than any of the original three.