Hi R. - This was almost exactly like a post I put up last year at this time!! When my youngest was born last June (she's now 14 months), my oldest was only 22 months. She did NOT take well to the baby being in the house and getting attention, crying, nursing, etc., so she escalated her terrible two's a little and basically was insane for the whole summer! It was awful, I remember like it was yesterday. Some of it is your son's age - the terrible two's are a very real thing. He'd likely be doing all of that anyway, but with the new baby, there is an added dimension. There is definitely jealousy, and kids want your attention above all else, even behaving well. I think, based on my experience, that you are doing everything right! One thing I'd do differently is spend more one-on-one time with my oldest if I could have that time back, but the demands of a newborn don't always allow that, unfortunately. Maybe when the baby is sleeping, you can use that time for your middle son and just do an activity he likes and make him feel special. Also, I found that lots of praise went a long way - when your son helps with baby care or is gentle with the baby, praise the heck out of him until you can't stand it anymore! It may not work immediately but he is storing it all up, believe me.
When he screams a ton after you tell him no, you might need to urge him to take a time out or quiet time until he's calm, then explain why you said no to whatever it was. He's just at an age, unfortunately, where he doesn't quite understand why you say no to some things that may be dangerous or unacceptable, but keep communicating and he will get it in time. I agree with another post who said maybe your husband could go to him in the middle of the night when he's screaming and just calm him down, talk to him, and get him back to sleep, so he doesn't think he can do that and immediately get your attention. Especially overnight!! I know it's all so tough with a new baby, but give it a few months and everyone will be adjusted that much more. And now that my oldest is 3, she is much more reasonable and I fell like I can actually get through to her rather than just be a discipline freak all the time. Hang in there! And have fun with your three boys!