Rocket is extra toasty and hit the deck hard (used almost all of the emergency crush core), but otherwise good
An earlier tweet says:
Falcon 9 will experience its highest ever reentry force and heat in today's launch. Good chance rocket booster doesn't make it back.
The narrator in the SpaceX BulgariaSat-1 webcast says at around
T -00:11:32 (about 04:56 into the video currently) that this is an extra-challenging, three-engine landing:
Our landing today involves some of the highest heating and structural loads on the first stage that we have seen to date, and it includes a three-engine landing burn. While it is still a secondary goal, this landing is going to prove to be extra challenging for us. But if we are successful, this will be the first rocket to land on both our East- and West-coast drone ships.
I'm fairly sure that burning three roughly ~650 kN Merlin 1D (throttle-able) engines to land a nearly empty rocket has something to do with the hot landing, and there must have been something about this launch that made the use of three engines necessary or at least the best option, but I don't know what it is.
Question: Why were three engines used for the F9 1st stage landing burn for BulgariaSat-1?
A short description of why the use of three engines for the landing burn makes the landing so challenging would be great as well.