Dan, I really don't think anyone here is arguing with you for the sake of it. I think they're more taking issue with some of the things you've said...
I appreciate that you haven't asked for advice on how to communicate with your girlfriend, but the reason some posters are doing so anyway is that you are likely to find it very hard to improve your son's behaviour unless you and your girlfriend are working as a team, discussing and agreeing on your parenting techniques. Also, quite honestly it doesn't sound as if you particularly love or respect her (referring to her as stupid, and saying that you need to be in control), so obviously people here aren't going to like that!
Back on topic - you've had good advice here, and there's only a couple of things I'd add. Firstly, try and see things from your son's point of view. It's difficult being three. You're old enough to have some physical size, strength and dexterity, and the ability to communicate your wants verbally fairly well, but you have very little real control over your environment or what you get to do or don't do. It's pretty frustrating. Also, you're only a little kid so you haven't got brilliant control over your emotions. You don't always even know what they are. You look to your parents to help you handle that bit - to name and manage anger, frustration, sadness, fear etc.
One of the most valuable parenting techniques is distraction. Three year olds have fairly short attention spans and are easily diverted onto new things. So when you see your son doing something you don't want him to do (or if you're able to anticipate this), quickly find something else to occupy him. It doesn't have to be anything complicated or amazing - we head off tantrums by eg spotting a dog walking past the window, having five minutes playing with a toy together, or if it's in a shop try challenging him to find the things you need to buy - "where's the milk? Can you see it? Oh YES, there it is! Well done! Now, we need a green milk today. Can you see the green ones? Great! I bet you can't lift it into the basket. Oh wow, you did! Awesome!". You get the idea.
That leads me on to praise and positivity. There's an old saying - an ounce of praise is worth a pound of punishment. And it's so true. You'll get SO much further by dealing with your son positively and encouragingly, rather than always getting on his case if you see him doing something you don't want him to do. Once he starts getting praised for doing things right, he'll start seeing the point in trying to do things right, and he'll do it more and more.
You said that one source of friction between you and your gf is that she wants to comfort your son after tantrums. You're right that you have to be careful not to reinforce bad behaviour, but on the other hand of course you must comfort a child who's genuinely upset. It's all a part of helping them learn to handle their emotions as I said above - and also demonstrating that your love for him is unconditional. You may be pretty pissed off with some of his behaviour, but you love HIM and you always will - and he needs to know that.
Finally - remember that your son loves his parents more than anything else. He desperately wants to make you happy and win your approval. Just make it easy for him by showing how much you love him and showing him that he can make you happy by behaving well. If you don't give enough positive attention then you'll see more bad behaviour - because your son knows that's one guaranteed way to bring you running. And then, at least he's got your attention. Getting told off isn't as good as getting praised, of course, but he'll take it if it's all that's on offer. It's better than being ignored.
Phew. Bit of an epic post. Hope it's helpful. Good luck. Come back and let us know how it's going. But please, be a bit nicer to and about your girlfriend. She's the mother of your child, she deserves your respect, and you're not likely to crack the issues with your son's behaviour without having her fully on board.