After a goalless draw last month, the Premier League side made no mistake in their fourth-round replay as goals from Juan Mata, Marcos Rojo and James Wilson booked a meeting with Preston North End.
Manchester United controlled proceedings at Old Trafford and could have won by a greater margin had their finishing been more clinical.
Van Gaal felt his side could have disposed of the League Two side with greater comfort, but was pleased to progress in a competition that he has given top priority to for the second half of the season.
"We have done what we had to," he told BBC Sport. "Every club in the FA Cup wants to win, we also.
"We have to beat Preston and that will not be easy, they won away against a cup fighter like Sheffield United, so it will be difficult for us to win in Preston North End.
"For the club, the fans and the coach and my staff, we want to win a title this year. The biggest chance is the FA Cup, but we still have to win four games to do that.
"Tonight, I don't think we played a very good match. But it is always difficult against a defensive team. We could have done better but I am pleased with the result."
Cambridge boss Richard Money left Manchester in positive mood after his side produced two encouraging performances, albeit while lacking the decisive touch to pull off a remarkable upset.
Things could have been much different had Tom Elliott's effort for Cambridge not struck the post in the opening minute, but Money was nevertheless proud with the effort from his side.
"I thought it [Elliott's shot] was in," he told BT Sport. "It probably wouldn't have made any difference, but he has done everything right.
"It was a good chance, not sure how much more he could have done.
"I think 3-0 is par for the course. We walked off the pitch with our pride intact.
"In the second half we have done ourselves proud. We gave everything we wanted to give, we have got to use it to keep going as a team and football club."
