I like Valkyrie's answer very much: show him counter-examples. I also agree with balanced mama, that his having concrete ideas about the differences between boys and girls is developmental. I'd just add one more thing, looking at it from your son's point of view:
He's noticed that girls and boys are different, and more importantly, that they are treated -- unless you're living in a very enlightened area! -- differently by the adults around them. I'll bet someone recently and oh-so-helpfully told him what those differences "are," and that this little bit of info seems to match what he sees around him, so he's incorporated it into his knowledge of the world. Girls play with dolls, Bella at the daycare is a girl; Bella plays with dolls. It holds up. He may even have gone further and incorporated it into his knowledge of himself. He is a boy; boys like trucks; he likes trucks. Yay! He has mastered the boy-girl difference. It fits in with what he sees of the world, and his world behaves in a way he can predict.
Now you are telling him that "not all girls like pink, or dolls, or flowers, and that some boys like pink, or dolls, or flowers" and insisting he accept that. I bet he loves you very much and wants to believe what you are telling him, but you are contradicting what he has just learned of the world, and rejecting his newly learned categorizing skills, the ones that work! And you not replacing them with anything! No wonder he's resisting your insistance, and is probably more than a little confused. Children at that age like a black-and-white world; something either is a thing or it is not.
What I would suggest is you validate his reasoning and reinforce his sense of identity without reinforcing western cultural stereotypes. Lots of girls do like to play with dolls, but not all, and some like to play with trucks as well. Lots of boys like to play with trucks, but not all. The people who make TV shows sometimes like to do it the easy way, so they show girls only doing things that most girls like, and boys only doing things that most boys like, because they don't want people who watch the show to be confused. The problem is that sometimes the children watching TV shows haven't tried to play with both dolls and trucks and don't know how much fun both can be, so they think they only can play with the toys shown on the TV shows. But real people like all sorts of different things, and we want to let our friends play with the types of toys they want to play with.
With my own child I would (and did) go further: He is a boy because he has boy parts, just like other boys, and he will grow up to be a man like his father. It doesn't matter whether he plays with dolls or trucks, he will always be a boy. (I would not mention trans-gendered people to a three-year old.) Girls are girls because they have girl parts, and will grow up to be women like his mother. (You may want to add here that girls' and boys' parts are private, and it's rude to want to look at them.) I recognize that not every parent wants to open this can of worms when their child is so young, but I think most kids (especially if they are around babies being changed) have already noticed these differences. In a few years he will outgrow the need for a simple world where everything obeys rigid rules, and be open to the multitude of counter-examples you are showing him, but until then, if you base his boy-ness on his physical body, he may find it safe to play with "girl" toys, and to let girls play with "boy" toys.
One more thing, if I may. There are plenty of TV shows that do not show the sexes behaving according to strict gendered stereotypes. (Caillou and Curious George spring to mind.) If the daycare must have TV on, can you have any impact on what shows are shown?