Three started to be worse than two, but in a few weeks, he came around, especially when we go somewhere. I told him my BIG rule: Care, Share and Listen and always ask for Permission. Pretty much, anytime he's in trouble, he breaks that rule in some way. I say it sing-songy, so he knows it's not a life sentence or anything, but that it is to be taken seriously b/c I tell him, "we're going to the store, I need for you to be good while mommy looks for things we need." If a tantrum starts in the store, I give him a warning, then if it continues, I pull him out of the cart and put him in time-out in AISLE TWO if I have to. I get a lot of dirty looks from people who think I'm a monster, but I also show my two boys that I'm the boss at home, in the car, and in the stores. I've also pulled over and put him in time-out on a safe spot on the side of the road or in parking lots. Now, he's 3.5 and when he starts to throw a fit ANYWHERE, I just say, "I see time-out right there," and point at a random spot in a store or on the road and his attitude changes quick. He walks next to the cart now with one hand on the cart and we ENJOY ourselves most places we go. Maybe once every two or three months I put him in time-out in WALMART or something, but not often at all. You'd be amazed at how willing employees are to help you, I've walked up to "LOWE'S" employees (and in other places) and said, (so my son can hear), "I see you have a time-out spot over there, do you mind if I use that?" Every single time, the employee of the store picked-up on my ploy and said, "sure, it's there if you need it," sort of thing. There's a Time-out is EVERYWHERE I go :)
We also use an "Angry Square" which we can take with us. I tell him, "It's OK to be angry about my decision, but you need to be angry on your square." It acknowledges his feelings, gives him permission to be mad, but sets boundaries for him to deal with it in a better way. While he's in his square, I ignore the tantrum, a few seconds later, he gets up and says he feels better. If I ask him if he likes his angry square, he says, "YES." There's a difference between tantrums for no reason, just attention and tantrums as a reaction to something that upsets a child. I never call him "BAD" ever, I just say, "It is not OK for you to act this way in the store," that's his warning, next comes time-out in aisle 10 for our family. :) good luck, it's a part of development; that they realize they can't always control their environment and they do the only thing they think will work. :) BTW, I've never had to leave a store, I refuse to let the child change the plans that we need to do; I let people give me dirty looks while my child is kicking and screaming in the store, sitting next to the pizza cooler b/c if I leave the store one time, I feel they will know they can control me with their tantrums and I'm not havin' it. :) A dirty look from a stranger once means nothing if I can get my child to respect me all the time in a matter of minutes.