Two stories which circulate at FSI:
The Fed's made a stop at Aspen the other day and checked all departing IFR traffic. And guess what, the only aircraft to fly legally out of Aspen IFR are tree-engine Falcons. All other twin jets departing IFR got busted for vioaling the FARs. Cuz there´s no way they could have managed the required engine out climb gradient. Maybe under certain condition they might be able. But I reckon they checked on a hot summer day.
Oh, and the Falcon 900 is the only airplane which can legally fly between St Moritz and NY non-stop. Now that must be quite convenient for some individuals with deep pockets.
I am sure there are a lot more examples which prove the flexibility of three-engine Falcons.