I have talked to her about calm down techniques

This won't work. During a meltdown, people aren't rational and “calm down techniques” ask for rationality to be proceeded.

I would recommend multiple points during these meltdowns. The first is meltdowns are a normal. It's part of the neurodiversity life. Your daughter have nothing to be ashamed of. She can't control them when they are happening. It will be a thing during her entire life. We can have influences on the triggers of meltdowns, but not on the meltdown itself. It's the old “Prevention is better than cure”.

Meltdowns are an answer to anxiety and stress. It's a huge release of energy to lower this anxiety. Anxiety is a general issue in Autism. We are anxious, as the world isn't shape for our neurology. She and you, the parents, and the teacher can list what she is doing and what's happening during the day. Also, hyper and hyposensitivities are triggers. So be sure to list them too. There will be some pattern, sometime more general and sometime more detailed. You will discover some triggers (not all as it evolves during time). It's not to avoid the triggers at all cost. It's to allow rest time. School for example use an astonishing quantity of energy (noise, light, peoples, talks, etc.). It's nice to have rest time when coming back from school to gain energy and lower the anxiety. If the noise is an issue, wearing something lowering it.

Now, when a meltdown appears. The first reaction is to stay calm. If the people around aren't calm, it will add anxiety and remember, meltdowns are a response to anxiety. The next steps are to not look at her, it can be perceived as predatory, don't ask question why it happens, don't speak of the consequences, again reasons won't work with a non-reasonable person. You will have to be reassuring by saying “it's fine to have a meltdown”, “it will pass”, “I understand what you're living, feeling”. It's to rebuild the self-esteem.

You can have a safe-space for her so when she has a meltdown, she can go there. It has to be sensitive friendly and “anxiety-free”. She can go there to lower the anxiety.

She is young but learning about autism is key. She will have to live in an autistic way to have fewer meltdowns, shutdowns and avoiding burnouts. It's to learn who she is, how she works, and to accept it. For example, she goes to a birthday party. She needs activities giving her energy and lowering anxiety before or/and after.