The word "pose" just makes me laugh, because it implies that any of my 3 would sit still long enough for me to get them all ready for a photo. It's usually more a matter of, "Ooh--you're together and no one is crying. I hope I can grab my camera and manage a shot before that changes!" I have great photos of each of my kids alone, but the ones I have of any of them together stink on ice, to use one of DH's expressions.
Here are a few of the ones I've taken that worked with a sitting baby. None of them are great, but maybe your kids will be more cooperative than mine!
Action Shots
Together in a big chair (I've done these with the baby and one brother, but the one I came across first tonight is the 2 boys. With the baby I have a big brother put an arm around her.)
Laying on their tummies
Big brother seated on something low behind the baby (Our 2006 holiday card, LOL)
Sitting in an older child's legs
We had a few done at Picture People recently that came out well. Lauren is crying so I can't get those uploaded right now, but one of the best ones was Lauren standing while Jason held her hands.