I've gone while 7-8 weeks and 26 weeks pregnant, and also taken YDD when she was 3 months and 4.5 months. How old will your other kids be when #3 is 3 months and when you are 3 months pregnant? 3 month olds are really portable, sleep a lot and are easy, especially if you are breastfeeding. Our trip when YDD was 3 months was among our easiest, and ODD's experience wasn't affected by YDD at all. at the same time, depending on the age of your middle child, I'm not sure I'd want to deal with the Terrible Twos and Tantrums either. All other things being equal, I'd go with a 3 month old.