I know you really want project advice, but it sounds like he's just vying for your attention over the two babies. I can't help putting in my two cents' worth on the attitude thing - hope you don't mind. After five kids, I learned that it's best to get on top of stuff now, while they're still young enough! Here are some ideas I think might help you:
(1) Sit him down at a time when he's not acting out - maybe during a meal time when you have his undivided attention, or while the babies are asleep. Tell him it's a LOT OF WORK taking care of three children. Explain to him that he is the big brother, and you need his help with this. That you expect him to behave so that your work isn't harder. Remind him that he is old enough to play by himself when you're busy with the babies. Promise him that you will still find time to play with him, but not when you are taking care of the babies. Tell him that you know he might not like this, but there are good things about being the oldest too - and list a couple. Tell him that you KNOW he CAN behave (list a time or two when he pleased you), and that you expect him to do that ALL the time (try not to laugh when you say that).
(2) Tell him that you also know that he gets bored. Tell him that the two of you together will put together a box that has toys he can play with or things to do, and that when he gets bored, he needs to do those things - not bug you. Ask him to help you make a list of things that should go in it - his favorite toys, etch-a-sketch, some crayons and paper, sidewalk chalk, whatever. I see the other moms have some GREAT ideas for play stuff. Use a paper & pencil to write down what he says so he knows you are serious and are taking him seriously. If he lists something that can't go in it, tell him that's a great idea, but it won't fit in the box, or it's the kind of toy he can't play with alone, or whatever. THEN GET THE BOX TOGETHER ASAP!
(3) Tell him that he's all set up now, and knows the rules. Tell him WHAT TO EXPECT if he breaks the rules. It shouldn't be "yelling," because yelling is still attention. I would strongly suggest that you tell him you will send him to his room to play every time he acts up, and that he will be able to decide when to come out himself - when he believes he can play without causing a problem for you. If he comes back out and starts again - immediately put him back. Do it quietly, without yelling. In my opinion, the best thing is to put one hand on his shoulder, and steer him down the hall to his room, saying gently - you need to be in your room until you can stop [whatever it is he's doing]. Do not argue if he tries to engage you. Just repeat the same line, then put him in his room and tell him he can come out when he thinks he can behave, close the door and walk away.
(4) MAKE TIME every week to do something, just you and him. It can be only 20 minutes if that's all you have - just as long as it is really about just the two of you. Even if it is only a walk around the block, or laying on the floor with him while he is playing his favorite game. And when you do - specifically tell him - "remember, I said I would make time for you! Well, I am!"
If you do these things pretty consistently and it doesn't work - then you need better advice than anything I've read in a parenting book or tried on my kids!
Good luck... and take care of yourself. Three kids can exhaust anyone...