My dd went thru a volatile stage right before her 3rd b-day--wild, violent meltdowns every day. I was crying myself to sleep every night trying to understand why this was happening. Turns out she was stressed out about turning 3 (we had been talking a lot about her bday), and wanted to stay 2.
Don't know if that is common at all, but I have heard it from at least one other person--so just know that the birthday itself can be stressful!
Other than that, 3 has been great. Much more reasonable here. Tantrums still happen, but are much more rare. I think the idea of becoming a "big girl" brought forth some new (or stronger) social fears for dd during her 3s, but she is conquering them at her own pace. Huge developmental leaps, too. Dd is begging to take a dance class, and taught herself to swim in the past 2 weeks (just threw her body into the water and learned by sheer determination

). For us, 3 is a balance of holding her close--cause sometimes she can be so little and full of needs!--and letting her go--cause sometimes she can be so independent and capable!!!