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*).
I guess takings would have been even higher had it not been a stinker. I wonder how many people would risk a 3rd movie though.
But yes, you can see why a studio would fancy it