I think if they plan on having Venom in the third film they will have to introduce him more in the second one. His name was only mentioned once in the first movie and most people probably didn't notice. They shouldn't have Venom and Carnage in the same movie though. Spidey had the alien costume for a lot of issues before it was revealed to be an evil symbiote, and it was awhile after that before it was revealed to be Venom. Most of the comic book nuts will want the secret wars movie where spidey got the costume before venom, then a movie with the alien costume, then Venom. It isn't really possible to have Carnage in the same movie with Venom, especially if it is both of their first appearences. If you really think about it a movie with Venom should be like four movies away, then Carnage a couple movies later. For all I know there are only going to be three movies. Carnage is like movie ten vilain, so prepare for a long wait, unless they don't do justice to him or venom
Your dead right there!! if carnage ever makes it into a spidey film - which i don't think he will(shame) it will be about the seventh film, there where over 100 issues between spidey geting the black costume and Carnage(8 years), that gonna take a couple of film to cover - but then again there where 48 issue's between black costume and venom!!