Edit: I didn't know that three boosters were supposed to land. That probably would've blown my mind if it had succeeded ^^
They setup the landings so that they are on a trajectory to hit the ocean, and only if everything goes right do they then adjust course to the landing pads. This gives them an additional failsafe which the center booster took advantage of here. Instead of crashing into an expensive barge in the ocean, it just splashed down into the ocean purposely missing the barge.
[1] is a diagram that kind of shows what this looks like for land based landings, but it's very similar for barge landings too.
And don't worry, they are going to be flying Falcon Heavy again soon-ish, so there's another chance to land all 3!
Probably a quad/octocopter - I know they've used one before for footage.
Third booster was supposed to land on a droneship - it was a near miss: https://www.youtube.com/watch?v=BXd5UHFuZVI
It needed to relight three engines, it ran out of igniter before lighting more than one.