I remember my mum telling me that I should have children in my twenties (she had me and my brother in her thirties too and was considered an older mother at 30 when I was born!) as I would have far more energy, but I didn't quite manage it. I'm sure that, sleep-wise at least, I would have been able to cope with babies far better at that age; I definitely need more sleep as I get older.
I don't remember DD1 and DD2 being too jealous of DS, although DD1 was quite jealous of DD2 when she was born and used to get up to all kinds of mischief when I was feeding her. I tried to give them as much attention as I could when DS was asleep, but he never used to sleep for very long during the day when he was little - and there always seemed to be so many other things to do (still are in fact; I feel constantly guilty about how little "quality time" I actually spend with the the DC).
It's difficult, I know - but it's early days yet and the older ones will obviously need a little time to adjust. I found the first few months if not years after DS was born quite exhausting (he wasn't an "easy baby" like DD2 had been, which I'd put down to me being more relaxed as a mother and so I was expecting DS3 to be equally easy - more fool me!), but it does get much easier as they get older. It probably still seems like a long way away at the moment, but once Samuel is old enough to play with the other two (or at least be entertained by/entertain them), I'm sure they'll have so much fun together. When they're all getting on and not quarrelling, it's lovely. Unfortunately, they also have phases where they squabble constantly - I know it's normal, but it drives me mad at times. Anyway, to cut a long story short, hang in on there; the older ages obviously have their challenges too, but I find them easier to deal with than the baby years.
Oh, and if your DD is anything like DD2, school will wear her out and she'll hopefully start sleeping better. DD2 is only at school for four hours in the morning so I was surprised it made such a difference, but she's definitely sleeping far better than she used to.