Sounds a lot like how my middle son behaved, we've done a lot to curb his behavior, but once in a while, the "ugly Michael" comes through. Negative attn is better than no attention at all. They don't really understand that's what they're doing, but if it seems to be working, they'll keep doing it.
First, don't play favorites (even if you aren't trying to play favorites, the extra attention your other children may be getting, is probably seen as "playing favorites" to your middle child. Even when modifying how you interract with your children, you still need to remain firm....and if spankings don't work...try 10 min. in the corner. Believe me, it's a worse punishment than a spanking if they are an active child!
Second, take time to spend one-on-one time with each child, even if it's just reading a story, or playing kick-ball in the yard. Something I've learned is to take one child a night, into the kitchen, and spend time cooking with them. I let them have the simpler part of the meal to make, and I give instructions, praise, and start a conversation.
Third, if your older child is able to do lots of things, then your younger child should get to do equally as many, even if they aren't the same. If your older child is taking a day trip with a church group, let your middle child go to a friends' house, or have a friend over, or go to grandma's, etc.
Find someone you are comfortable with leaving your youngest with, so you can spend one on one time with your middle child, like going to the grocery store together, alone, or taking her to DQ for a treat. These kinds of things don't cost a lot, but definitely add to her emotional well-being.
I hope my advice helps. I am a mom to three, and my middle child made our life hell for about 5 years, until we got help to learn how to stop it. If you need to talk, feel free to e-mail. I have plenty of other suggestions, but didn't want to take up all of your time.
S.
p.s. Something else I learned. Start small with taking things away. I found that there were many times my middle child would sabbatoge fun family outings, like to the fair, or park, or a baseball game. I've learned to try to not threaten taking away those types of things, because 9 out of 10 times, we'd all lose out! :(