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): His MCU solo debut. 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.