We travelled to the mid-West for Thanksgiving with our 7-week-old. I was prepared for the worst, but he was an absolute dream for the whole week (including flying both directions). He got worked up once when he was tired and wanted to nurse/sleep and it was "extended family picture time" (i.e. chaos) - I put up with that for a few minutes and then said "Enough!" to the picture-takers and took him away to a quiet room; he calmed down immediately.
Could your husband be a little bit worried about successfully executing his role as "travel daddy" to your three-year-old during your trip? You're going to be busy with baby quite a bit, which will leave your husband to do a lot of the things for your daughter that you might otherwise take care of. My husband is a wonderful dad, but he doubts himself from time to time, and sometimes ends up undermining his own ability to do a fantastic job.
If you can, pack a DVD player and stickers and washable markers (or those that only color on special paper) and other quiet activites that your husband can supervise when you need baby time. I hope you have a womderful trip!