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).
Jensen Ackles for MCU Venom!
Spider-Man (2017): The film would star Vulture as the villain, hired by Kingpin to stop the new hero, and take Peter back to high school. Eddie Brock would be introduced as well.