DTs (both boys) are almost 21 months. I also have a 3.3 yo DS.
One of my twins is constantly biting the other two. He will do anything to get a rise out of them - taking their toys, pushing them over, pulling hair.
All 3 tend to fight over wanting the same toy. We have plenty of toys to go around yet they can scream and wail over one tiny little Duplo dog for half an hour!
I'm getting very stressed and anxious about it all. We physically separate them when he lashes out. I know he does it partly to get a reaction, partly because he can't fully articulate what it is he wants to do.
When does this behaviour get a bit easier to manage?
They all get plenty of exercise and fresh air, as much as we can.
I'm finding the constant fighting is wearing me down.
It doesnt really. My girls are 4.5 now and can fight like cat and dog. Hair pulling is the method of choice now. I mostly ignore them and let them sort it out, but unfortunately yours are too little to do that.