The Chronicles of the Invaders by John Connolly & Jennifer Ridyard

Conquest: Book 1, The Chronicles of the Invaders (The Chronicles of the Invaders Trilogy) - The Illyri have conquered Earth.  The Illyri invade with the military and once a world submits to the Illyri, the Diplomats take over.  The Nairene Sisterhood makes up a third Illyri power.  These three groups of Illyri do not get along with each other.  Therefore, while Lord Andrus is the Illyri governor of the British and Irish Islands and the most powerful Illyri on Earth, the Diplomats and the Sisterhood believe that the military has failed at subduing Earth and are plotting to remove Andrus from power.

Paul Kerr and his brother Steven are part of the Resistance in Edinburgh fighting to regain freedom from the Illyri.  On her 16th birthday, Syl Hellais, Governor Andrus' daughter, sneaks out of her father's stronghold, Edinburgh Castle, to explore the city.  While out, the Castle is attacked and the Kerrs save Syl and her best friend Ani.  Later, Syl returns the favor, and because of this, she becomes an outcast.  The Kerrs, Syl, and Ani find themselves depending on each other to escape the Illyri who want to kill the Kerrs and send Syl and Ani to another world.

Empire: Book 2, The Chronicles of the Invaders - Paul Kerr and his brother Steven have been conscripted into the Illyri Brigades.  They are sent on an exploratory mission when their away team is attacked by an unknown species.  Escaping that, they find their main vessel attacked by an unknown group. Fighting off that group, they find themselves and their small contingent running from everyone.

Meanwhile, Syl and Ani are being kept and indoctrinated by the Nairene Sisterhood.  Ani is accepted into the elite Gifted group, while Syl hides her powers from everyone.  She soon discovers that the Sisterhood has many secrets; and she intends to find out what they are.

Dominion: The Chronicles of the Invaders - Paul, Steven, and Syl are beyond the Derith Wormhole where they are learning about the species that reside there.  Meanwhile, back on Earth, the Illyri have let the Others take over and the few human survivors are doing what they can to survive and to learn how to rid Earth of the invaders.