Christopher Nkunku from the spot, an own goal and Noni Madueke struck for the Blues.
Maresca said: "Very happy. When we lost, for us the result is always important because we are Chelsea but in this moment, the main focus cannot be the result. It's to analyse or judge the performance and to see how we improve. In my personal opinion, from Wrexham, Celtic and today, we continue to improve. This is the most important thing.
"We struggled against Wrexham for many reasons, we struggled against Celtic for many reasons but there were many important things we improved. Today, we continued that line. Since we started, what we do in training has to happen here in the game.
"What I didn't like against Celtic was the habit we have defensively. For instance, to hold the line very high when there is no pressure on the ball. This is suicide. You cannot hold the line very high when there is no pressure on the ball.
"We are talking, we are working. It's a habit from years ago. I don't know. There are things that we need to improve defensively but the way we pressed against Celtic, going man-to-man against a team that is ready to start the championship, we had to be brave every time."
On Nkunku, he also said: "I know Christo from years ago. I loved him before I joined this club. In the way we want to play, he is the ideal player. He can play in the pocket, in the nine, on the wing. In this moment we are using him in the pocket and he is doing very well.
"The reason he is playing every game is because we know that he is coming back from injuries and needs minutes. He is doing very well and also game after game he is getting better.
"You can see today; Kiernan is speaking a different language to his teammates because he knows exactly when he has to move wide, attack behind, that was one year with us. Christo is improving that with us. Enzo's first day but he is going to improve."