Sorry OP, it can be really tough, can't it!
You do seem to be rejecting quite a few suggestions a bit quickly, so I assume you are feeling pretty stressed and tired of the situation. Before worrying about your specific problem - are you getting enrough time away from your children? This sounds a bit like a "you can't pour from an empty cup" situation. Grandparents nearby to take eldest to the park? DH to take them both to soft play for a couple of hours? What support so you have so that you can be fresh and the best parent you can be? It's worth putting as much thought into that as the specific issue.
I do empathise. My twins are about the same age as your eldest. New baby is just coming up to 4 months in our case, and we also have a 7yo. Dear God do the 3yos make some days tough. Both poo'd in their pants this morning, and although the first one I think it was just that he didn't get there in time I'm sure the other one did it deliberately!!!! They are too rough with the baby, and I'm certain that its occasionally deliberately. A mixture of curiosity and jealousy I think. Very tough to handle.
Although its really hard, please do try to giv the eldest lots of time and positive attention. They really need it at that age. The youngest will be fine with a bit of neglect now and then, honest! Playpen or other safe area and some toys can buy you 15 minutes with with bit of luck.
I find mine much easier to manage away from home. At home, they will pester the baby of given even the slightest chance. But at playgroups they are much more interested in their peers. I also generally find 3yos to be less overwhelming when outside. Once a week or more we do a really long walk with planned stops - stop on a bench for a snack, stop at a playground for a run around etc. The youngest gets freedom at the rest stops!
Otherwise,try and focus on activities that your eldest can really enjoy and the baby might get some benefit from. Made some Angel delight with mine this morning - we took it inturns to stir, they had to carefully pour it into their bowl, had a blunt knife for chopping a banana in, few sprinkles tondecorate the top. A 10 month old could have a silocone spoon and bowl and feel very involved(making a right mess!) While your eldest did it "properly" because he's a bit boy. Remember that attention to you child is the success criteria, it doesn't matter if the desert looks even remotely good. The boys enjoyed eating theirs afterwards and I guaratee DH will find a smile and some compliments for them when he gets home even if it does resemble a bowl of slop with glitter on top.....
Always remember "this too will pass". Honestly, they get MUCH easier. My eldest gets cross at her brothers ocxasioanlly, but I certainly don't have to watch her at all times.