Did you leave out a 1? She is 3 right, not 13? They do this, they manipulate (not the negative implication) stories to get their imaginations going. She isn't doing it to trick you. You can talk to her about why the truth is so important and give examples of a lie and the truth This is a developmental stage they use it trying to figure out how far their imagination stretches. Wash her mouth out? Really? That is just crazy. You don't expect her to know what 22+3+4+9 is do you? Same thing, this concept is not something she will rationalize.I hope you don't hit her like that. I don't know why you would even bring that up... that is extreme to slap a child in the face whether it was condoned in the past or not, especially for something that is as normal as a 2 year old wanting to do things by themselves.
She isn't doing it as a filthy habit, the "I didn't do it", if she is truly doing it to get out of trouble then you need to re-evaluate your punishments... your punishments shouldn't breed sneakiness and if they do they are too harsh. My daughter is almost 3 and I couldn't imagine sticking soap in her mouth because she stretched the truth, my heart would break just watching that esp at a young age where this is a great creative development. She said yes confidently b/c maybe she thought you were talking about another day or she thought it was today and it wasn't, my mom thought she took her medicine one day but didn't. I think you are placing too much expectation on your 3 year old.
I agree with Michelle Q and started wondering if you punish harshly or punish for unrealistic expectations and I do think that these kind of punishment considerations are a slippery slope sometimes. There are tons of healthy ways of teaching and consequences that don't involve traumatizing her taste buds.
And yes, there are kids that have a reaction to the soap in their mouth and die. A punishment should never cause a child to be anywhere near death. A time out, toy taken away, etc will never cause death (unless there's some crazy neglect in hand with it)
So what happened: Good for you, I hope that's for real. I didn't get from your original wording that you did it, but I did add it in my post just in case because I do not think it's okay to slap or wash a child's mouth. It's good and I hope your really do take a step back when you get mad, I know how that is. There are a few times where I had to go into another room and just chill out for a minute.