well shes smart but u shld tell ur friend if u thinks it right

3 year olds should understand the difference between right and wrong, which is basically the same as telling the difference between lying and telling the truth. If the child is a chronic liar - they probably have had lots of practice and mom and dad need to be a little more insistant with follow through to discover the truth. Most kids lie for attention or because they think they can get away with it. As long as mom and dad are willing to go that extra little bit - the lying will likely stop after a while. How long it will take is up to how strong willed the child typically is.

Yep. Human beings start to tell lies around the age of 3 years old. It is thought that this has evolved from a survival instinct and it is normal - they all do it. Of course, they are not any good at it and will look you in the eye with utter conviction and lie their face off. It is not to be confused with adults who lie because at 3 years old, a child is not capable of complex deception. On top of this you must remember that a 3 year old child has very little idea of how to behave, you have to teach them absolutely everything because they are not born knowing right from wrong - they are born as blank slates. All kids lie - all of them. When she lies it's your GF's job to teach her about what lying is and why you shouldn't do it and so on. As far as the child is concerned, she has just says one thing to get another thing and it goes no deeper than that. It's no good just saying 'stop telling lies', she won't understand what the word 'lie' means or if she's done it. This is why toddlers are really crap at doing it. As with anything, your 'don't lie' speech will have to be repeated over and over again and she will gain an understanding - BUT it will be a 3 year old understanding of the concept and that is worlds away from an adult understanding.

Not properly. They might have a dim idea but they are too young to know the difference between wishful thinking and hardcore lying.