The Gunners trialled the new formation against Middlesbrough last week, when they ground out a 2-1 win at the Emirates.
REUTERS
Arsenal beat Manchester City 2-1 after extra time
It saw Arsenal claim back-to-back wins for the first time since January ahead of crucial games against champions Leicester and north London rivals Tottenham.
And Koscielny, who played at the centre of the back three at Wembley between Gabriel and Rob Holding, believes the new system has made Arsenal harder to beat.
"There was a tactical turnaround but we have been working on it for 15 days," Koscielny said.
PA
Arsene Wenger has played three at the back for the past two games
Invalid email
We use your sign-up to provide content in ways you've consented to and to improve our understanding of you. This may include adverts from us and 3rd parties based on our understanding. You can unsubscribe at any time. Read our Privacy Policy
"The system suits certain players but it's also good for the whole team. I think we have also found more confidence in this system with three defenders.
"We are more solid. Commitment and determination also helped us to win because over the whole game we played as a team and fought hard."