Mostly, I think that your son needs to not play independently at their house. Personally, I don't think he IS old enough at three (if I read correctly). But clearly, if he can't follow the eating rules you set for him, then he can't go there. That's not really his fault, i's just a lot of pressure for him to turn down a yummy snack offered by a friend. He is too young to make good choices on his own. Food is one, but there are others. If the parents are that lazy with their own kids, there are probably a lot more hazards at their home then you want your son exposed to (it could be merely poor supervison to a loaded gun in a closet). My five year old is just strating to play at friends' houses without me. Even if we agree that we have different parentling styules in that regard, this particular house doesn't sound like a good home for you little boy to be in unsupervised.
But if he keeps playing there, you could try talking to the mom and dad honestly. If that makes you uncomfortable, then you could say that he is coming home sick and has a sensitive stomach or reacts badly to preservatives or whatever. Have him bring snack over - fruit and cheese and a water bottle. At three, I was still bringink snacks to my daughter's playdates, so it doesn't seem out of line.
I think it is great that you care about this. Once in a while junk, you kind of have to deal with, I guess, but if it is a regular thing, then you certainly have to teach your son that you have certain expectaitons for his health and his behavior and you expect him to make good choices. You need to teach him this stuff, since you are trying to teach him to survive in a world full of choices. So if he keeps playing there, give him strategies to make the right decisions. And if tha tmeans telling him to come home at snack time, do that too. Good luck.