HELP! very jealous 2 year old

I would suggest that you not only time him out but ask him to express how he's feeling. Give him a crayon and tell him to draw a picture of how he feels. He may not be able to express what's going on so he acts out in frustration. Give him lots of praise for appropriate interaction. He may be feeling displaced of course because this could be overwhelming. These three children may have a pecking order that he may not have been included in. Made a rule that there is no hitting period in your house. Have them come up with other alternatives to express frustration. Sit them all down and say something that we are a new version of our family so we all have to come up with some rules. You may be surprised what they come up with. Get all the ingredients out to make cookies and have them focus on that while you talk about the other issues. Divert their attention to something that creates a reward they will have to have patience to get. Explain that the rules are for the time it takes to bake the cookies they need to find something to do that will include all of them and do it without any hitting. Small steps.