It is not at all unusual for a 3yo to be volatile. A stressful or confusing situation, like currently with covid (or having a new sibling or something) may exacerbate this behaviour but it can also simply be a normal stage of development. If you think about it, reacting against chastisement isn't actually an abnormal thing to do: it's an age where children need to become more independent of their parent but have very little actual power and no negotiating skills. An older child can say "Yes, I understand you're worried but I think it would be good for me to try this and I promise I'll stick to any rules we agree". A 3yo has no more sophisticated resources than stamping his foot and shouting "I'LL DO WHAT I LIKE WHEN YOU'RE DEAD!"
The best thing you can do as a parent is to stay calm and cheerful, carry on doing whatever you intended, take things away from him if need be, but not show him that he makes you sad or worried. That is too big and scary for a 3yo: he shouldn't have that power!
Doesn't matter how gentle and beautiful his mum is: she has taken on the job of being the strong unshakeable rock in his life. If she doesn't feel it she needs to fake it. Think about a good headteacher: they almost certainly don't feel that in control all the time, but they pretend they do and that gives them control.
A sense of humour helps enormously and can be used to distract him as well as keeping her sane (at the same time, this is an age where they're developing a sense of self so he really shouldn't feel she is laughing at him).
He needs to feel that he cannot break mummy, that she will ensure that he doesn't get to do things he mustn't, but that every day is a new day and she never stops loving him.
My dd was very difficult around this age. No covid at the time, but having a little brother was unsettling to her and she did try to hurt not only me but the baby as well. Had to shut him in a room with a high latch when I went to the loo because they really weren't safe to leave together. We got through it, she adjusted, they had a lovely relationship, we had a lovely relationship.
Your dd/DIL needs to hang in there, most likely it is a phase, she will come out the other end and he will carry with him the memory that mum keeps him safe even his own emotions.