D.D.
My older children were 5 and 3 when my twins were born so I know how tough it is trying to balance the needs of everyone when everyone is still needs so much.
I can't offer much help on the girls since basically it's trial and error to figure out what works but I do have a suggestion on how to get your son to act like a big boy. I use to point out all the times that my 3 yr old could do that the babies couldn't and that made her proud of herself. When I'd pour her cereal I'd made a comment like 'Boy you are so lucky because those babies can't eat cereal. They don't have any teeth. How about you?' and she'd tell me that she was a big girl and could use a spoon. I'd always take the time to point out all the ways she was bigger, stronger, and more talented than the babies.