Hi S.:
I'm not sure I have any advice yet - more like sister solidarity. I'm having the same trouble with my three-year-old. We are always late, weekdays and weekends.
He tends to throw tantrums in the supermarket. I have often sat on the floor (or stood) right there in the store just holding him for up to ten minutes until he calms down and stops kicking. Needless to say, I often have scratches and bruises all over after that. My threats are usually - if you don't behave, no Diego and no Dora. If he continues, I add in all the rest - no Little Bear and no Max and Ruby - all his favourites. Then I tell him that he will watch Jeopardy with Mama for two days. Sometimes he calms down and says that he will behave. I have a new one, too. The other day, he told me he would like to watch "Fresh Yogurn". I said "Fresh Yoghurt"? No, Fresh Yogurn. He tried describing it to me - blue and orange. I later found out that it was Backyardigans. I haven't watched it with him yet, so I don't know if it will become one of "our" favourite TV shows.
Yesterday, he was so upset that he threw up. I stood holding him in the grocery store until he calmed down. He was calm right up till the end of shopping. When we got home, I sat him down and we had a chat about what had happened in the store. I asked him why he had been screaming like that. It took him a while, but he eventually told me that it was because his shoe fell down (he ended up throwing up on it, anyway, and couldn't put it back on). I said to him that all he had to do was to tell Mama calmly and quietly. I made him repeat twice quietly "Mama, my shoe please". Then I said that was all he needed to do, rather than screaming and kicking. I acted out what he looked like doing this, which he found very funny. And I said, none of that. Instead you say... and I made him repeat it again.
I don't know if he will remember this. I will test it this week and weekend, and keep reinforcing it, and hopefully, it will work.
N. M.