How do I stop my 3 year old from harming my newborn?

Bless him, he probably feels left out. Maybe try doing activities with him, without the baby. Or involve him helping with the baby.

I had this problem and they do grow out of it eventually.. I feel at 3 probably come out of this phase sooner rather than later.. sorry I don’t have any advice as I was at my wits end too because I had newborn girl boy twins and a just over 1.5year old son and it went on for ages the “hitting/scratching/throwingtoys phase”
I knew my then 1.5 year old was jealous because of the divided attention and took a while adjust but I felt every time he hit them I’d tell him calmly and firmly that’s not the right thing to do and kind soft hands etc lol before you know it they’ll be playing with each other, at the time it’s a lot but it will get easier.. my twins now just turned 3 and i feel that my year old boy twin would most probably do that if i had a newborn 😅 also good to try and distract them for example give them a toy they haven’t seen for ages/activity to do xx

Sorry you’re going through this! My firstborn (although wasn’t trying to hurt baby) did struggle a lot of jealousy in the beginning. The thing that helped me the most was sitting down with him and showing him photos and videos of when he was born and when he was small. And then I’d tell him stories about what he was like, and what we had to do to protect him and basically guided the stories to deal with whatever the issue was if that makes sense. Then he seems so much more protective of lil bro. I don’t know if that’ll help 🤷🏼‍♀️

I've been sort of trying to prepare for an older child/ baby transition and some of the advice I've found or been given so far is as follows: buy a gift for the older child and explain that it is a gift from the baby as a starting point, try to include the older child by having them help out with the baby in a hands off way such as helping pick out outfits or grab wipes during a change etc
Otherwise we're just planning to basically helicopter parent the children when they're in the same room, try and redirect the behaviour into something else... Haven't actually gotten to try any of this out so I don't know how helpful it is 🤷🏼‍♀️ but that's what I have so far