3 Year Old Troubles

[LittleOwl1987]

me And my partner are experiencing real difficulty at this time, my 3 Year old is becoming really hard work, he’s aggressive, we have a newborn, only 2 months old, and can’t leave him in a room with 3 year old, I’ve walked in to him trying to stand on him, sit on him, put things over his face, intentionally trying to hurt him, I’ve tried involving him with changing his nappies and wiping him down etc, but even when try and include him, he’s so rough with him and intentionally wiped him trying to hurt him,

Of course you can't leave them alone together!! This is basic. Your three year old is feeling the most insecure and threatened he ever has been, as his survival is still totally reliant on his care givers. And you are now both providing care to another child. He's wired to not be happy about that!
You firstly make sure he can't hurt the baby. Baby in a sling so they are happy and safe, and this also allows you to play as normal with your three year old. One of them has to come with you if you leave the room for any reason, even incredibly briefly. Make sure he puts toys down before coming to see the baby - cars turn into throwable weapons very fast. And then lots and lots of calm modelling of how to interact. Teach him if he doesn't want to or feels mad or anxious or funny inside he tells you with a short phrase (short and the same each time because he'll be getting overwhelmed and unable to remember or say it otherwise, such as "I need space"). Then you know he needs help staying calm and you can move the baby away, and give him a cuddle or a toy to play with instead.