The suspect was detained at the scene at Molly Road, Derrylin, Co Fermanagh, and transferred to hospital for treatment prior to his arrest.
Detective Superintendent Jason Murphy said: “At this time we believe the fire, which has claimed at least three lives, was started deliberately.
“We are continuing to thoroughly examine the scene along with our colleagues in the Northern Ireland Fire and Rescue Service.
“I would renew my appeal to anyone who was in the Molly Road/Doon Road area this morning to contact police.
“We are particularly keen to speak to anyone who was in, or travelling through Derrylin prior to the time the fire broke out at approximately 7.10am.
“Please call 101, quoting reference number 179 27/02/18.”