Alejandro Garnacho, Casemiro, and Anthony Martial scored the goals that saw United start their Carabao Cup defence in style.
Manchester City had to dig deep after Rodri was sent off early in the second half of their 2-0 win over Nottingham Forest.
Manchester United were comprehensively demolished by Brighton after goals by Danny Welbeck, Pascal Gross, and Joao Pedro.
Bruno Fernandes completed a stunning comeback as Manchester United came back from 2-0 down to win 3-2 against 10-man Nottingham Forest.
Julian Alvarez scored the winner as Manchester City eased past Newcastle United with Phil Foden pulling the strings in the absence of Kevin De Bruyne