ds3 used to get that alot, not quite as often now but I still notice it occaiosnally.
It used to scare the life out of me because we were referred to th hospital about a heart mermur when he was tiny.
Anyway, it wasn't anything linked to his heart but just when he was cold and because he's so teeny (your dd3 is weeny for her age too, isn't she) it seemed top show more because he hasn't got a lovely chubby coating or flesh like lots of other toddlers. He's lovely but is a bag of bones!
Also my mum said it can be a sign of dehydration. Not entirely sure where she got that from though...