S.H.
You just instill boundaries/rules etc. And if they scream/tantrum, then so be it. Be consistent. A child WILL deflate on their own... after tantruming. Just let him deflate. If you try to "reason" with a kid when they are screaming or having a tantrum, they are not able to listen or be rational. So just wait until the storm is over. Then, reiterate your reason for saying no etc.
My son is 3.5... and that is what I do. Unless he REALLY is in need of something or help or understanding from me. I will say "I know you are frustrated... but...." and then explain my reason. After he is done being upset... he will most always, come to me and say "Sorry Mommy..." Then I praise him for that.
And no, 3 year olds do NOT have fully developed "impulse-control" yet, nor total understanding of their emotions, nor how to express it or cope with it. Hence, they tantrum.
But if you give in to every little scream/tantrum... then that will be the "habit" they learn, that you "teach" them.
One time, my son wanted chocolate. From a box that i had bought for myself. I let him have one. He asked nicely. Then he wanted another. I said no, he already had one. He screamed and tantrumed. I said "You can scream but I still will not give you one. That's all. I said only 1." Then he screamed more. I let him. He calmed down, then asked again. I said no. He screamed. So then, I took the box of chocolate, and emptied it into the trash can. Right in front of him. That's it. He didn't expect me to do that. I told him, that was my box too... but he couldn't have it all. And so now, it was just all gone. No more. It really taught him... that Mommy will not give in.
All the best,
Susan