As a mom with three boys, who are three, six, and nine years old, my experience has been that ages three and four are the hardest ages (so far.) The parts of the brain that help control impulses and calm emotions are just still very undeveloped, but their emotional range and desires are in full force! At these ages, when they are losing it and having a full blown tantrum, they are not really in a teachable frame of mind. So what do you do? 1. Identify with the feeling: "You're really angry/annoyed/frustrated."
2. Give the directive to stop the behavior "Hitting is not OK" or “No more throwing, please.”
3. Change the situation (either remove her, distract her, or get her onto something else).
4. Talk about the behavior when she’s in a calm state of mind.
I want to focus now on this last step. Conventional wisdom says you have to address misbehavior immediately, or the child won’t remember. But the fact is that a child won’t hear what you’re trying to teach if she’s tantruming. So yes, address the issue as soon as you can, but only when the child is in a calm and receptive state of mind (and it might even need to be the next day).
You can do it in a way where she feels like you two are just talking, not like she's in trouble. Just something like, "Hmmm. You know yesterday, you got so mad. You hit your friend, then you kicked mommy. I wonder why you were having such a hard time. . . Do you have any ideas about why?" This way, she's being given the opportunity to get practice reflecting on her behavior and getting into the practice of self-insight. I know you won’t usually get great answers at ages three and four, but you’re laying the groundwork.
Also, ask her what she could do differently next time she gets so mad. Ask what she would like for you to do to help her calm down. Asking questions like these will help her continue to learn about relationships, planning ahead, the need to regulate emotions, expressing herself appropriately, etc. It will also communicate how important her input and ideas are to you. She’ll more and more understand that she's an individual, separate from you, but that you are very interested in her thoughts and feelings. LOTS of opportunities here for wonderful experiences that are great for a growing preschooler!