I'm trying to figure out what is a stage and what is some abnormal behavior that I need to address specifically. This is why the 2nd kid has it so much easier... you realize it's just a stage and just shrug it off rather than drag yourself over the coals of guilt about your parenting and fear that you've screwed up the kid perminently.
The books say that three is hard b/c the kid realizes she is not a baby anymore and is excited about being grown, but is also afraid of being grown. I suppose it does involve much unknown, and the unknown is nearly always scary. One of the books mentions that toddlers are "small, slow, and clumsy" which is the genesis of much frustration with themselves and the world. I can see that. Also, they feed on structure and predictibility. It's the way they feel safe in the world. Hence our meltdown yesterday, which involved my pulling the car out of the garage before trying to load in kids. She absolutely freaked out. Turns out the car must be in the garage in order to climb in and fasten one's carseat. Who knew? Seriously, a complete meltdown.
It's hard to know how to respond. Sometimes what she's asking for seems so small and irrelevant that why not give it to her? If she feels out of control in the big big world and pulling the car back in the garage will help, then why not? But there are other times when I feel like she's trying to test me and see how far she can push, in which case the best approach is simply to hold the line. And then there are the times when I just don't want to.