Rising baseball star Blake Bivens has thanked God for the "most wonderful" wife and son in a heartbreaking tribute days after they were slain alongside his mother-in-law in a horrific triple homicide.

His wife Emily, 25, their 14-month-old son Cullen, Emily's mother Joan Bernard, and their family dog were found dead at their home in rural Virginia, US, on Tuesday. 

Emily's younger brother Matthew Barnard, 18, was arrested in connection with the deaths after being found running naked through the streets and attacking passersby. He has been charged with three counts of first degree murder.

Bivens, 24, a pitching prospect for the Tampa Bay Rays, said in the moving Facebook post that his "heart was turned to ash" when he found out about their deaths. 

"My life as I knew it is destroyed. The pain my family and I feel is unbearable and cannot be put into words. I shake and tremble at the thought of our future without them," he said. 

In the touching tribute to Emily, he described her as "the best wife and mother this world has ever seen".

"You made me into the man I am today and you loved me with all of my flaws. You brought our precious baby boy into this world and made our family complete. Your love and kindness changed countless lives, including mine," he said. 

Turning his attention to his young son, who had only just celebrated his first birthday in June, he called him his "reason why". 

"My sweet little boy, dada loves you so much! I can't breathe without you here," he wrote.

"I finally understood what love was when you were born and I would have done anything for you. You have changed my life forever, you are my reason why. I long to hold the both of you again in heaven. I'm so glad you are with all your Great-grandmothers now, I know they are eating you up."

He added, "This earth did not deserve either of you; you were just too wonderful to comprehend." 

He went on to praise Joan as the "best mother-in-law anyone could ask for" and said that he and his father-in-law, Skip, would "get through this together as family". 

"We will not let the devil win!" he said. 

Despite the heartwrenching loss, Bivens still thanked God and said he was trusting in His plan. 

"Thank you God for giving me the most wonderful family in the world! I've been blessed beyond belief," he wrote. 

He added: "I am comforted by all the messages and well wishes. I'm not sure what is next for me, but I do know God has a plan even though I can't see it."