Ah honey not really sure what to say but I understand it must be so hideous for you, Archer has some pretty serious tantrums and he is only 2and a half and I am struggling to cope with him and no2 on the way so I don't know how you are managing with a bigger boy!
I am making some headway though, although I have found that things have to get worse before they get better. I used to give in to him just for a quiet life (think this made things worse in the long run) but I have started to be really strict and no means no now. After quite a fight he will usually give in.
Don't know if any of these things will help but this is what I've been doing.
I threaten to take toys away if he misbehaves or tell him that he won't be able to have his friend to play/see Nana/watch Timmy Time to try and head off tantrums way before they escalate. A few weeks ago I let him go on one of those rides in town, he was really happy but then had a belter of a tantrum when it finished and I woulnd't let him go on it again! Now he never gets to go on them! I always tell him they're broken (does that make me a bad mother?!)
Another thing that has really helped is I have become a complete and total food nazi! I never really beleived in the whole food/behaviour link but I am now convinced. Archer is never allowed squash unless its after a meal (water only) and snacks are very strictly monitored and rarely allowed. Its 3 meals a day now. Before I was always completely relaxed about the food thing but since I have been really strict he does seem to be better behaved. It took a few days for him to get used to the new rules but he now seems to have accepted them and rarely asks for things he's not allowed.
I also find if A is tired he is soooo much worse. Even though most of his mates the same age don't have naps anymore I strictly enforce naptime and he has at least 1 hour if not 2 in the afternoon. I find this doesn't effect his nighttime sleeping at all (woulnd't matter if it did as he is behind a baby gate and can't get out, we usually do wake up in the morning and find he has trashed his room though but we have started removing everything from his room that can do damage and he now just has a big box of paper and old magazines to 'play' with!!).
I expect you have already tried all these things but just thought worth mentioning.
If you have tried everything I would ask your HV for some advice. I find mine pretty useless most of the time but worth an ask. Also might be worth googling to see if anyone else has any bright ideas.