i have learned with my daughter when she gets upset that stroking her somewhere with the back of my hand and telling her gentle over and over again helps to calm her alot quicker than anything else i have tried.
My daughter used to have meltdowns a lot before we started figuring some things out. most of her meltdowns, some of which would be 45 minute screaming fits, were mostly sensory related (some of which I had figured out and others I hadn't) or we violated a ritualistic behavior we didnt' realize she had. Or she wanted to communicate and we didnt' understand her and she was just so frustrated. That was right before we started therapy when she turned 2.
Have you talked with the parents? Do they have therapist(s) that might be able to show you things? Does (s)he have sensory issues? Does (s)he communicate?
Can you give us a bit more info.
Can you tell us what country you are in.
How old are the triplets.
Have you been given any indication of how where the child is on the spectrum eg. from mild to severe.
There will be a reason for it, even if you can't see what it is at the moment.
Can you give us some examples of what is upsetting her. Try to think of what has just happened before she got upset/angry. For example is it when you move her, or give her a toy, or take one away, or tidy up, or singing, or group activities etc.