I feel the same. My 6 year old is doing it too, but my 3 year old only just started.
All I can say is relax & be consistant. The last few days Bri has really be getting to me & I've been very short tempered compared to normal, which I think is making her worse.
Just try to be calm & not yell. It just gets you worked up & stressed if you lose it with them - It won't bother them too much though!
Do you have a naughty spot? Maybe they can have a corner each. But then if they are like mine, they will sneak to each other!
Mine are always worse when we are out. They know I can't put them in the corner or smack them in public. So if we are at say the drs I will make a naughty chair. Or just let them know they are going to the naught spot when you get home & stick to it.
If they want something at the shops say no & even if they kick & scream stick to your guns. My girls have thrown so many tanty's in shopping centres, I just ignore them. If we are walking I keep walking - they won't let me get too far without coming after me. & if we are at the checkout or something I just don't look at them.
It's not easy, but if you give in just once they will think that they can do that every time & get what they want.

Thats what I do.