Story whose final line is "Let there be light!"

Question

I read this series around 5-10 years ago. There were two parallel universes, ours and the universe of demons, and much of the series revolved around attacks in which demons created gateways to cross into our universe to kill people and wreak havoc, and had to be stopped by human magicians.

Three youngsters (one guy from modern times, one from a few decades ago, and a girl from ancient Celtic times, all with funny names) end up being the key to the war when it's revealed that they are the three components of a mythical superweapon. These three are the only point-of-view characters in the series, each book being told from the PoV of just one of them. At the very end, they

destroy our entire universe so that, after killing all the demon masters, they can rebuild it and make sure everything happens as before up until the very last moment.

As they begin recreating the universe, the current PoV character (the one from modern times) says something like the following, the very last lines of the series. (sic on the quotation marks)

"In the beginning, there was nothing but darkness. Then [name] said, 'Let there be light!'"

And there was light.

Coolio!

Answer 1

This is the Demonata series by Darren Shan.

The series does revolve around three children (quoting from Wikipedia):

The series centers around three protagonists: Grubitsch "Grubbs" Grady, Cornelius "Kernel" Fleck and Bec MacConn. Although they meet each other at various points, they are from separate times; Grubbs lives in the present day, Kernel in the 1970s and Bec in around 450AD.

And it is about a long conflict with a parallel demon universe:

The books detail their fight against the evil demon master Lord Loss, his many demon familiars and the mysterious Shadow, who promises to destroy the human universe and to even stop death. Together with The Disciples, the main characters thwart the Demonata's attempts at world destruction multiple times.

The text you quote is from the tenth and last book Hell's Heroes:

Clearing my throat, to a chorus of good-natured groans from Kernel and Bec, I chant solemnly. “In the beginning Grubbs created the heavens and the earth, and everything was dark. Then Grubbs said, ‘Let there be light!’”
And there was light.
Coolio!