At my wits end!!!!!!!!

Okay parents.....I am at my wits end with my three year old. Either she is possessed by the devil or she is "just" the middle child and she is going thru all that entails. I was the baby and the only girl, so I have no idea what she is feeling. So as her mother I am trying to understand and adjust, but it just isnt working at all. Everyday is a new ugliness episode, and I just shake my head and wonder why and what I can do to fix it. Hopefully  by putting her in preschool soon, this will help alot, but who knows? Is it me she is butting heads with, because it got really bad when my husband had to suddenly work a second job. But the weird part is, he goes when they are asleep. How does she even know he is gone? Strange!!!! PLease anybody out there with any advice would be great.......like I said I am at my wits end!!!