Lots of good suggestions here.
Now, per your 7 year old and your 3 year old. That is 4 years, difference.
My kids are 4 years apart as well.
In order to be fair, to them, I take consideration of their ages and development, per their respective ages. I do not EXPECT them to be the same. They are not. 4 years is a an age spread.
My daughter, who is the older one, knows that her younger brother is not the same as her, thus, she cannot "expect" him to understand things like she does. How does she know that? Because, I spend a lot of time talking with her about age development and how, she and her brother are developmentally not the same. So her expectations of him, are fairer.
But I never expect my daughter, to parent him. That is my job.
Then for my youngest, my son, I teach him about communication and problem solving and speaking up and saying his feelings. To us. This enables him to facilitate... what is going on in his head or emotions. And I teach him coping skills etc. I did this with both my kids from when they were 2 years old. If he is frustrated, then he knows he can say it or express it. And, this helps a lot.
And each of them need time, to themselves or just with me or my Husband. I have taught them "quality time" too, and to say when they need that, from me. If I am forgetful about it. So they don't get frustrated.
And I don't expect them to "share" everything, or to have to be doing things together all the time. Teaching them about "personal space" and to say, their needs, too. To me. So that helps a great deal. As well.
Otherwise, siblings can get on each other's nerves.
Or if one child is more "controlling" & rigid or territorial than the other, then there will be problems. I know, I have a sibling that was like that.
Try to teach your kids, that family means looking out for each other, and having each other's back. That is important... they have each other. And you are all family.
Teach your kids about the word "compromise." When my daughter was 2 years old, that is one of the first words I taught her. And then for my son when he was 2. And how this word, is important for family. And to get along.
This is hard. Your daughter may be expelled.
The teachers have witnessed this.
And the other parents are upset.
The school has done what they can, given their "rules."
Maybe get your daughter an assessment.
I also know of a girl, my daughter's classmate, that has a younger brother. They have a tenuous relationship. But it is because (this girl vented to me once about it), SHE has to always mind her younger sibling... babysitting him and playing with him, and doing so many things with him. Because her parents both are very busy and work etc. But so that means, she has no fun, of her own. And she gets deeply frustrated by this. And they both fight. She is just a girl. And being a parent to her younger brother, is too much for her. But her parents think she is older, thus able... to do it. And that that is her "job" as an older sibling.
But the younger sibling, is hard to handle. I have seen it myself.