Remove all expectations. The Hangover Part III is not the movie you expect.
After an explosive first movie, and a disappointing retread of the same formula (that is unsurprisingly ignored throughout Part III), the Wolfpack returns for its grand finale. This bittersweet conclusion brings with it a mash-up of genres and tone, while avoiding the use of an actual Hangover to drive the plot. It brings a significant amount of change to the formula, which many critics have not been comfortable with.
If you are to view the general consensus, critics have not been the biggest supporters of this change to say the least (average Metacritic score of 38). But while I can understand their problems with this complete departure from their original formula, this is an issue of perception rather than with the film itself.