The two side boosters of the first stage have flown before. One launched the Thaicom 8 communications satellite in May 2016, and the other lofted a Dragon cargo ship to the ISS for NASA in July 2016, according to SpaceX.
View from the VAB of launch complex 39A.
When Falcon Heavy lifts off, it will be the most powerful operational rocket in the world by a factor of two. With the ability to lift into orbit nearly 64 metric tons (141,000 lb)--a mass greater than a 737 jetliner loaded with passengers, crew, luggage and fuel.At this time, SpaceX has said the upper stage has performed two of its three scheduled burns with the last scheduled around 5 hours after the last burn, or around 9:30 EST tonight.
Falcon Heavy's first stage is composed of three Falcon 9 nine-engine cores whose 27 Merlin engines together generate more than 5 million pounds of thrust at liftoff, equal to approximately eighteen 747 aircraft. Only the Saturn V moon rocket, last flown in 1973, delivered more payload to orbit.
EDIT 2/6/18, 1921 EST: From Spaceflight Now: "Speaking to reporters at the Kennedy Space Center, Elon Musk says the Falcon Heavy's center core did not survive its descent to SpaceX's drone ship in the Atlantic Ocean."
Hopefully, more later.