I have two boys, ages 5 and 3. The 5-year-old has ADHD, and shows defiance to authority at times. I have read the other responses and agree with most of the disciplines. Be consistent. Get down on his level and make eye contact. Tell him it is unacceptable, have him clean it up, and do time-out or privileges lost. Talk to him about feelings and using his words. You know all that.
I also see the need for individual attention. AND, kids crave structure. (Example, food only at the table. There should not be food in your bedroom!) Set specific times for specific activities.
Things that have worked for me. First, give praise 5 times for every negative comment. Hard, I know. Get creative. "I like that you put your shoes on right away. That was a very big boy thing to do." "You did a good job picking up your toys." Even, "I like the blue shirt you picked to wear today." It needs to be from some specific action your boy did, a decision he made. "That was nice of you to share your toy with your little brother." You get the idea. It acknowledges that they have the ability to make good choices, and it lets them know you notice.
Two, you say you are busy. Get your kids to help. My boys love to vacuum and dust. They help make biscuits, too. They can knead dough like no one's business!
Three, work on relationships with your kids' friends' parents, and arrange playdates. You can send two of them off and have special time with one.
Four, take time out to play with them. Get down on the floor and play with the train set. Put some music on and dance with them. Go outside, and play Ring-Around-the-Rosie. Write a story together; they can draw the pictures and you can help with the words. Then, Dad can read it a bedtime. Or, ask them what they want to do.
Again, give them choices, and praise them for making good choices.