Welcome to the forum. Those are wonderful ideas. Thank you for sharing. Hopefully, we will hear more from you!
Love the idea of the calming jar. If you can use that before she melts down it should be helpful.
Main point is certainly the consistency. When she melts down - put her in a short timeout. Don't try talking to her, don't try to reason with her until she has calmed down and is out of the time out. This will take a while to be effective. She has learned what works for her and it will take time to change that behavior. Essentially, the rules for behavior modification are that there must be immediate, short, consistent consequences
And as Jsprings hinted at working with and teaching them how to respond will also help. I would also look into buying "Cool down and work through anger" or "When I feel angry". This is part of a series of books aimed at 4 to 7 ( I think they will work with this age group too if kept real simple) year olds and meant to be read to them at night (several times) and then practiced. Kids do need to be taught how to deal with anger. You do not try and use these techniques while she is screaming. But once she stops or later on in the day - you can refer back to them or pull the books back out.
You can find them here - http://www.amazon.com/Cool-Through-Anger-Learning-Along/dp/1575423464/ref=pd_sim_b_5
Hope this helps too!
There is some thought that rages like this are caused by seizure-like activity. It sounds like she's in a rage and she doesn't even fully know why - she just is in a rage. She tries to think of something to rage about but when given that, she's still in a rage -
Massacheussets General Hospital Neurology unit had a webforum several years ago where they discussed rages in small children as possible seizure activity.
In regards to the car, they sell seat belt covers that make it impossible for the child to undo it themselves. My daughter has autism so I am quite familiar with these types of tantrums. On the bus (she is in kindergarten) they have a special booster seat for her that has it's own buckles. She cannot get out of that seat on her own. Since it is a safety issue I would just make it impossible for her to get out on her own.
For the tantrums, those can be tricky. If possible, you should put her in time out and ignore them. I know that this is not always possible for various reasons (ie throwing, hitting the wall, etc). My daughter has a service dog who has been trained to lay on top of her for deep pressure. She seeks this out when she's angry and he is often the only one she will let near her in fits of anger. We also have weighted blankets that help calm her down. You will have to try out different things and find what works with her. Talk to your pediatrician too for some ideas. Maybe you need a referral to talk to a developmental pediatrician.
This is normal for 2 and 3 year olds to "test" boundaries. I cannot stress enough that you HAVE to be consistent. No exceptions whatsoever. And don't give in to her by switching mommy/daddy attempting to do the tasks. Although it's a different person, it doesn't show consistency.
At my centre, we have a jar and it's called a "calming jar". This jar is clear (small peanut butter jar with the lid taped shut) with the childrens favourite cartoon stickers on the outside of it. On the inside we filled it with water and dyed one pink and the other jar blue. We put glitter and beads inside. When we feel a child is in need of the "calming jar" we say to them "Johnny, I see you're a little bit frustrated. Do you want to tell me why you're frustrated?" (Can be replaced with mad, sad, tired, etc). Giving children words to match their emotions actually helps them calm down and they feel like they're noticed.When you're stressed, they're stressed as well. If they're trying to tell you, be patient and listen it makesa huge difference. If they don't tell you or they cross their arms and look away you can offer "Well I have something that you can use until maybe you're not feeling so frustrated. When you are done, you can put it back and we can talk about it." Giving the child something to do on time out will make it more effective. Think of children as sponges, they retain so much information. Now that they have the jar they see the sparkles and wonder "Well why does the bead move faster than the sparkle?" etc.
Give them 3-8 minutes with this jar, go over and say "I'm sorry that you felt frustrated. I know it feels bad inside when you're frustrated. Do you want a hug?" Showing them love after something like this is critical. It teaches them it's okay to have these feelings and no one is mad at them for it.
Also, try finding songs that have to do with activities you're struggling with.
"This is the way we sit in our seat, sit in our seat, sit in our seat. This is the way we sit in our seat, all the way there.
This is the way we buckle up, buckle up, buckle up, this is the way we buckle up so we stay safe
This is the way we raise our hand, raise our hands, raise our hands, this is the way we raise our hands because we're staying safe"
That tune right there (as well as Farmer in the Dell) can be used for so many songs you can make up (like I just did for buckling up). Try to make extra time in the morning. Tell her "Okay. You're going to buckle yourself up now today, but you can ask me if you need help" if she tries what she normally does, say "Nope. Mommy is going to help you now because you asked for it, and we need to be safe. Maybe you can try again tomorrow." Sing the song while you do it. Consistency is key.
Hope this novel of an answer helped!