Skye,
Marydean has given some great advice.
You have alot on your plate with two youngsters and a newborn too. One thing I though of is maybe her behaviour is escalating because of your new addition?
Another place to look for help is the Triple P parenting program. They help with positive parenting. They run short courses all over the place. Maybe it would be good for you and your hubby to go to some of these?
Alot of foods contain things that would negatively affect behaviours, not just junk foods. Have you noticed she gets hyper after eating anything in particular? Sometimes it's oranges or other fruits that can do it. I know alot of breads have additives in them that aren't very good. There are more but I can't think of them. A dietician could help. And possibly a naturopath.
Medication definately has it's place for some children, but I would try all other avenues first before medication. ADD/ADHD/ODD drugs are serious stuff and can have detrimental affects also, so I would try the other things first. Though sometimes it is a combination of things that work the best.
Good luck hun. I hope you find an answer.