'I'm joking' and 'I'm kidding' both mean 'I'm not being serious'. They have the same meaning. The only difference is that 'joking' is standard English, while 'kidding' is colloquial/informal. 'Teasing', which is often transitive, is always directed at a specific person. When we tease a person, we are trying to get a reaction out of them by playing upon that person's particular weakness. For example, if you know that your friend is scared of spiders, you might tease them. So, if you say "I've got a spider in my bag" ( when you haven't), you are joking/kidding about having a spider in order to tease your friend. Teasing does not have to be the same as joking, and it can be either kind or cruel. A good friend of mine here on italki knows that I'm not very good at maths, so he often makes reference to mathematics in his posts to me, as a way of teasing me. This is friendly teasing. But if children regularly make jokes about a fat classmate's weight, this is cruel teasing.

While the 3 words can have different meanings, in this context they all mean the same thing. EG A: Where is my chocolate? B: I ate it! A: What!!! You ate it? I'm going to kill you! B: woah, calm down, I'm only joking/kidding/teasing. It's also worth noting that joking and kidding are intransitive while teasing can be transitive. IE "I'm just teasing you" is ok but "I'm kidding/joking you" is not okay, you would have to say "I'm just joking with you."

tease： 1. to laugh at sb and make jokes about them, either in a friendly way or in order to annoy or embarrass them 2. to annoy an animal, especially by touching it, pulling its tail, etc. 3. to make sb sexually excited, especially when you do not intend to have sex with them kid： 1. to tell sb sth that is not true, especially as a joke 2. to allow sb/yourself to believe sth that is not true joke： to say sth to make people laugh; to tell a funny story，to say sth that is not true because you think it is funny --------------------------------------------------------------- What is the difference? It depends on what you really want to express. --------------------------------------------------------------- I hope that answers your question. If you'd like to practice speaking, I'd be happy to help you develop a study plan. Feel free to look at my teaching profile and class offerings and send me a message if you think I can help you.