Lots of good advise here.
I always make a point of squatting down to their level, I place one hand gently on his shoulder, look into his eyes, before I talk. I ask him why are you doing this. Mostly he doesnt know. or It is fun. Then I calmly tell him he has to stop doing it, and I tell him WHY. I give him the consequence. Like,
"You have to stop bouncing on this chair because it is very wobbly, can topple over and you may hurt yourself." or,
"Dont swing from the banister, I know it is fun, but if you lose the grip, you will fall down the stairs and get hurt"
"Please do not put 3 dvds into the dvd player at once, there is really only room for one, and if you put more in, the dvds will be damaged and the dvd player may break"
Then after I have explain, we will either tidy up the mess together (or he will just be looking), if there is a mess, or I will avert his attention to something else that is fun, such as "look, here is the book about animals, lets us look in it for a little".
Also, I try to be outdoors as much as possible, park, walks, playground, they are bouncing with energy, and kicking a ball with them, is really great fun.
And yes, boundaries should not be broken, not even once, because that is settign up a bad example for the future. And I dont give in to whining and moaning. A constant "mummy, ICE CREAM, sob sob, scream, ICE CREAM" will not be rewarded with ice cream, as then they know exactly how to get an icecream next time... moan moan, whine....
I dont promise them sweets or treats in advance, I dont mention it. But there is usually a pudding at our house, but it is not a reward, just a part of the meal that we eat when we have had our dinner.
Good Luck. You have taken an important step in recognising what sort of parent you want to be. I used to be like you, and I bought two very good books, one simply called Positive Parenting. I dont have them as they are boxed up in the garage (just moved).
Also, if you DO have a bad day, and you end up shouting and screaming, dont be too hard on yourself. It happens. BUT be prepared to be extra understanding the next couple of days. A child who is no longer accustomed to such behaviour will feel very insecure and needs a lot of reassurance. I have not yelled at my six year old for at least a year, but the other day I lost it, and I could see a pair of big round watery eyes, a shaking little hand, and quivering lips. Of course, I was guilt ridden. But I am only human, and what can I do? Apart from telling him, I am sorry, mummy overreacted and behaved very bad just now, I love you very much and I will NOT yell again. THen stick to it.
At three, they are also able to think about mummys behaviour and decide whether they like it or not. I lost my temper with my 3 year old too recently (massive stresses at the moment) and I asked him to go and lie down in mummys bed. I went in to him 5 minutes later when I had calmed down and he said "Have you come to say sorry, mummy?"
When you do feel yourself shake with frustration, when you do feel you are losing yourself, take yourself out of the situation. Let your mind go blank. Breathe. Go to toilet if you are at home, just sit, breathe, try to be calm, try to rationalize it, and think, they are little children, I dont want them to be hurt and upset" Go out there and go out with them.
Oh, this was long. I shall stop now. Good Luck!