Oh heck no, you're not alone! There's a moms of 3 yo support thread on the Childhood Years forum, I'll have to bump it and link you to it...it's really helped me.
Here's what I do - and I realize that there's a wide spectrum of GD people here, so I know that not everyone will agree with my routine, but it's what keeps me sane and keeps me from yelling.
When he does the, "BUT IT IS READY!" thing, I respond one more time, "No, honey, it is not ready, and it will be ready in about 15 minutes. You may either help/go color/play trains until it is ready." Then, I just stay quiet and take deep, long 5 second in and out breaths while he melts down. When he is approachable, I will lean down to hug him or just smile at him, or whatever, but I don't waste any more energy engaging him with discussion or reasoning.
If he's truly in freaking out mode, I will offer him a glass of milk or juice or a small, healthful snack, because when he's acting that way, he's usually either bored or hungry. If he refuses, then I tell him that he may certainly continue to holler and yell, but that he needs to do it in his room. If he refuses, I pick up and carry him in there, and set him in his bed. I tell him that it may be helpful for him to spend a quiet minute in his room, playing and just taking a break from what's frustrating him. I tell him that when he's ready, I would love some help with dinner/help him get his crayons, etc. And I walk away.
The key for me staying calm is doing the deep breathing, and trying to make the hollering/screaming/crying fit background noise. If he's shrieking to the point where I'm getting frustrated or angry, that's when I remove him to his room (or my room or wherever is comfy for him but away from me).
Three is hard, mama.