Worldwide box office takings:
The Hangover: $467.5 million
The Hangover Part II: $581.5 million
So yeah, i'd say that's why they feel a third go has merit. Whatever you think of the sequel, you can't deny that the series is a guaranteed money spinner, we're just lucky that the producers seem to realise that they can't keep it going forever and are ending it with the third film rather than running it completely into the ground with annual sequels (*cough* Paranormal Activity *cough*).
Ever heard of Friday The 13th The Final Chapter?
Soon followed by "A New Beginning" and about another 5 after that.