Venom is huge, his character takes time to be born. They need to advertise him. If they have Brock and the symbiote in 3 (not Venom), this would make an awesome cliff-hanger for people to go see 4 (and make 4 one of the first movies to make the fourth film better than the rest).
Put the black costume in Amazing Spider-Man 3, use the crashed space shuttle for its origin. Introduce Jensen Ackles as Brock, the climax will have Spider-Man lose the symbiote in the tower, then post-credits we see Brock as Venom.