I'm not David Cross, but I can tell you that he did not like playing Ian in the third film. He stated it was one of the most unpleasant things he's ever done, though he highly respected Janice and Ross and how they treated him.
The links above (and some of the links on those pages) say it all. One of the reasons that I remember was that he thought the third film was just a large commercial for Carnival Cruise lines. He also didn't like the fact that he was forced to wear the pelican suit himself (instead of an extra) despite times when he wasn't speaking. The real reason he agreed to act in the third film anyway was for the money to pay off a debt.