I'd like to help you with your question. It does seem that you have your hands full with 3 children who have different ranges of abilities. I'm glad to hear that you have identified the learning abilities and styles of each of your children. As you have witnessed, each of our children require a different level of intellectual stimulation. While the older two seem to manage their behavior well, your youngest is taking a far different path.
The easiest mistake you could make here is to say that its the intellectual and learning styles that separate these kids. But that's not the case. What you have here is a youngster who behaves badly as he has figured out that if he is headstrong, argumentative, etc. he is getting attention. Sure it's negative attention..but it is attention nonetheless. Same at school - his attention-seeking gets him at the front of the class or back of class.
What needs to change is that you get back in control. I would encourage you to get the following book on parenting:
Parenting with Love and Logic by Foster Cline and Jim Fay.
This is the very best parenting book on the market! You can buy this on-line through Amazon, etc. or on the Love and Logic website. The website also has fabulous parenting tips. I have used this book personally and with hundreds of clients. It is a life-saver.
Essentially, the book teaches you - in an simple, clear way - how to take back the power in your house. Right now, your young son is calling a lot of the shots - right? That's like the tail wagging the dog. Understand.
With Love and Logic - the rules are simple, the limits are clear. Mom & Dad are in charge. They make the decisions and kid follows.