TripMom, my dd does that, and she does not have to contend with triplets. She has no excuse. I've kind of started to notice a pattern, though.
First, sleep. If she's not getting enough sleep, she has no self-control and gets angry fast.
Second, I can't get too involved in their relationship. It's easy for me to see ds1 as a victim, but he does dish out occasionally and provoke her. And as much as I hate it, the more I try to advocate for him, the worse things are. I think he feels more like a victim, and she feels more resentful of him.
So, when things like this happen, I just calmly and neutrally repeat the rule. For example, dd hits ds1 and snatches away something he's playing with. These two things are violent and are against the rules, so I say, "Hey, no hitting, no snatching. Dd, if ds wants that back, you need to give it back." She'll say, "Here, ds, want this instead?" And the poor little sucker always says yes, so I move on.
Anyway, if I were you, when he scares them I would say nothing to him, unless you can turn it into a game. If he's already made them cry, I'd say, "Oh, that was scary for you! Did that scare you?" but I'd stay kind of blase about it. I think even 2.5 year olds have learned to manipulate the drama a little bit. And I don't mean that in a bad way, just that they're interested in the interaction and are learning the rules of relationships. And you don't want them learning to purposefully demonize their brother, right? For the hitting and pushing, I'd just get down on his level and calmly say, "No shoving!" If you have any idea about his motive, you could mention a possible one to give him the benefit of a doubt, too.