There are many reasons for the delay, one of which is Elon Time™, but the previously mentioned improvements to F9 and being able to launch many payloads with it instead is also among them.

As to when it will fly, I expect by the end of the year. They have finally begun static firing actual rocket cores recently, including the heavily redesigned (and another reason for delays) center core.

This is part of it:

“Falcon Heavy is one of those things that at first it sounded easy,” Musk said. “We’ll just take two first stages and use them as strap-on boosters. And like, actually no, this is crazy hard, and required a redesign of the center core, and a ton of additional hardware. It was actually shockingly difficult to go from a single core to a triple-core vehicle.”

http://spacenews.com/spacex-to-launch-falcon-heavy-with-two-...

It has literally been built and tested. The hardware is ready and just waiting for the launch pads.

The major reason why Falcon Heavy was delayed for so long is that they were able to make major improvements on the F9. It would not have made much sense to work on a more complicated rocket when there was so much potential in F9.

Falcon Heavy was developed for a pittance compared to the SLS. Even if its delayed another 3 years the SLS would not make a lick of sense.

I was pessimistic a few months ago but they've got Falcon Heavy cores on test stands now so it shouldn't be long now.

SpaceX has managed to get such a big boost to the F9's performance, they can already do much of what they originally intended to do with Falcon Heavy. Is it really going to be cheaper to loft sats with a reusable FH when a disposable F9 will do the trick? I'm skeptical.

Delta did it so I'm sure SpaceX can do it too. But I think it was more difficult than they though to fly three rockets in close formation.