Sounds like a 3 yr old to me. Especially one that's a middle child.
So, I'm thinking the 5yr old is in school and there are things you have to do with him for school. And he's the older brother who is your helper and has quite a bit of responsibility. The 1yr old is still in diapers, on a bottle, and you are lugging him around a lot. The 3yr old? Kinda in the middle. Not a baby, but not a big girl yet either. She's not used to not having the attention since the baby came along and now there's another one on the way.
I'm not judging you, I'm actually rather jealous. Had my husband and I met when we were younger and I was healthier we'd have had a house full. ;-)
I would try to make sure that she gets her own time, too. Believe me when I tell you I understand how whining gets on your nerves. My 8yr old is going through this AGAIN!! It seems to go in cycles for some children.
And remember that anything you feel or think they are sensitive to. Some a lot more than others. I'm having kind of a bad time right now and I think my son is feeding off of that. Hopefully as I get better so will he.
problem with whining and crying insecure borded or attention