I'm really baffled why a 3 year old would say that doctors were men and nurses were 'ladies'.
It cannot possibly reflect the reality of what he has seen
Sometimes it only takes one strong voice to convince a young child about stereotypes. 
My 3 year old DS has said similar things after spending time with one particular boy at his childminder's. This boy is aged about 6 and comes for a couple of hours a day of after-school care, and often says things like "girls can't be doctors", "cars are only for boys", etc. Unfortunately, DS hero-worships him a bit and takes everything he says as gospel, even when it contradicts his own experience. Hence, DS has told me "doctors are men" even when all the doctors he's ever seen have been female. 
It's hard work deprogramming him.