Capricorn 76 - your posts are fantastic, whether or not those who are blinded by their own sanctimoniousness will be able to engage with them.
I grew up in a rural county in America that actually had a law on the statute books banning African Amercans from the city limits of the county town until it was superseded by civil rights laws in the early 1970's. Believe me, when I was 3, I understood that some people were more "important" than others because of the colour of their skin even when no one had to directly tell me so.
Three most definitely is NOT too young to engage a child in discussions about difference-not-meaning-bad-scary-or-less-important, whether that applies to sex, ethnicity, religion, disability or any other factor. If you or any other parent choose not to have those conversations because you think they are too young, you find it uncomfortable or don't know how to go about it, imho, it's the same as saying you don't think keeping your child from growing into a bigot is important. I actually sense from this thread that some parents would be quite pleased if their children DID grow up that way.
I've said it before, but if a person stomps on your foot by accident, not intending to hurt you, it will still hurt the same as if they meant to do it. One can hurt someone else by saying something out of ignorance, but that doesn't excuse it. If you then try to justify what you've done and minimise the pain you've caused someone else, well in my book that starts to cross from "I didn't know it was hurtful," to "I don't care if it hurt you, that's your problem."