First of all I think challenging behaviour is normal for an 8 year old and having a go at a younger sibling is also normal.
So why is he doing it? He is too young to explain to you what is going on in his head. Although he understand some of the consequences of his actions he still doesn't fully understand that what he is doing is driving you mad. That is why your adult reasoning with him doesn't work. ( I am not saying that you shouldn't have boundaries either)
But what he is getting from you is a reaction and attention.
If HE can't do anything to stop ( as you have tried charts) then it really only leaves you to try and modify his behaviour.
First of all I don't think you should react to any negative behaviour ( I know it is bloody hard when he has thumped the little one) but if you yell and he yells, you all end up shouting louder and every one gets even more upset until the next time, and then it begins again. Shouting just doesn't work.
So, whatever the provocation is, appear CALM. Count ten. Pick your fights and only comment and sanction the more serious ones. Let minor ones go, it is important to be consistent.
Get him to help you with jobs around the house. Make him feel important and loved. As he appears to be suffering from jealousy issues.
Devise strategies where you do things that are FUN with him and reassure him that you love him. We all love our children but can still not like them much. We can easily get into a spiral of disliking them because of the phase they are going through.
Is there any way you can have time alone with him and try to get to like him again? Try and keep the children apart by giving him something to occupy him at flash point times and then use the time between the three year old going to bed and his bedtime, to play cards, or do a jigsaw, or dance to a piece of music, with him or do anything positive.
You can praise him lavishly when he wins and have nice rewards.
Most children want to be approved of by their Mum. Have you ever watched the Super Nanny programmes? She always says the ' fun ' has gone out of the families life when she is faced with siblings that fight and a despairing Mum?
You are far from being alone tryhard100 It is not easy. Honestly we have all been there. Hugs