ok, I just posted a long answer to your question, and this site lost it. I will wait overnight and see if it shows up. Till then I just wanted you to know that I saw your post and will get back to you. Oh, one of my many suggestions was to also post to the Child Behavior forum which I am also the CL. We have several involved mothers who also give excellent advice. Its link is - http://www.medhelp.org/forums/Child-Behavior/show/64
His primary doctor could do an initial evaluation and discuss next steps with you. It's likely that they will refer out to an ADD specialist. You could check out ATTitude Magazine's Diagnosing ADD guide which lays out the typical steps to getting a diagnosis. Also check out the organization CHADD.
The problem is that it is difficult to diagnose a child that young because clinical standards say a child must be diagnosed in two or more settings. If his only setting is at home a psyc will likely not diagnose as ADHD. Here is a real good link on that whole situation.http://www.additudemag.com/adhd/article/2488.html?utm_source=eletter&utm_medium=email&utm_campaign=May
If you are trying to get a 3 year old to look you in the eye while instructing them (especially if they have just been bad) or repeat commands - its just not going to happen with many of them. And, part of the problem may be that you are “talking” to him too much when he should just be going into timeouts - but more on that later.
There are some things that you can do that will help.
First, if you can afford it - get him into a day care a couple times a week for several hours. It will give you daughter a much needed break. But it will also provide a setting where experienced people can compare him to other kids. Which means he then can be accurately assessed. However, make sure it is a daycare that has kids of the same age and is one that allows the kids to move around.
Now that the weather is better - get him to a park or a fenced in field. Take a small soccer ball, a tennis ball, a frisbee, maybe a small airplane glider, and have him chase them. In other words, run the heck out of him. It will make a big difference at home. If he has a trike, take him somewhere that he can ride and ride and ride.
Make sure he is getting the right amount of sleep at night. Lack of sleep really messes up a kid.
A good diet is always important.
Buy the book,"SOS Help for Parents," by Lynn Clark. It gives you a very effective system using timeouts that will help change his behavior. Most people who try timeouts do it incorrectly and give up. This system works. To change behavior, you must be consistent and immediate. And even then it can take up to three weeks to see the result. This book explains all of that.
Finally, do realize that his sister may have “spoiled” you. I remember that raising my son was very different then raising my daughter (she got even when she turned 14). A three year old boy can be a force of nature. So its a bit hard for me to say if he has any of the things that you are fearing. That is why getting him involved with other kids is important. The first link I gave you will help you with the whole ADHD thing.
I hope this helps. Please post if you have any questions. Best wishes.