The first three books in Lori Wilde's bestselling series, Texas Rascals.

Keegan:

When a mysterious stranger shows up on Wren Matthew's doorstep, sick and delirious—the timid high school teacher can't leave him out in an ice storm. Not at Christmas. Even though she's sworn off men, empathetic Wren has little choice but to let him. And never mind that electric chemistry surging between them.
Former cop Keegan Winslow has a dark secret and the last thing he wants to do is draw sweet Wren into his sordid world. But she's given him a warm place to land in the raging storm and a kindness no one has shown him in a very long time.
But Keegan knows nothing can come of the love he's found on her dairy farm in this small Texas town, because staying here will put her very life in danger.

Matt:

Deputy Sheriff Matt Forrester was determined to discover who was stealing cattle from Savannah Markum's ranch. But as the man who'd never stopped loving her, he yearned to protect her—and the child who should have been his son. Just as Savanah should have been his wife...and would be—if Matt had his way.
Savannah's busy days were filled with single-handedly raising a toddler and running her ranch. Her lonely nights were filled with longing for what might have been...if Matt hadn't shattered her heart. Now Matt was back and Savannah had to decide whether to turn Matt away or take a second chance on a forever kind of love...

Nick

Pretend to love, honor, and obey that infuriating Nick Nickerson? Impossible! But policewoman Michele Mallory was stuck with the assignment—and a hunky new "husband"...
Disguised as newlyweds, Nick and Michele set out to investigate the Triple Fork Ranch in Rascal, Texas. Acting the loving couple by day, they attempt to ignore their nightly temptation. And wonder what will happen once "Assignment: Marriage" is over.