If your 1 yr old is in a carseat, she may kick the seat in front of her. The height and position of the carseat seem to make it easy for kids to kick the seat, even if they're not trying to.
If that's the case, I'd put your older child and your DH in one row and you and the baby behind them so if baby kicks the seat, she won't be doing it to a stranger.
We're a family of 5 and we've always put DH & the older 2 kids in one row and youngest child and me behind them. However, the next time we travel, DS won't be in a carseat, so we'll probably go with all seats in the same row at that point.
Yes my DD1 will have a car seat. (Ideally we would rent a car and need it for that.) I think a car seat would be easier on the plane, but I had not yet thought that far ahead. Seems there is about = for both arguements with 3 and 1 and 2 and 2.