for my own understanding of the 900, I found this explanation for the trijet.
".... nearly every commercial jet design has two engines instead of three — yes, a few very large models have four.
The same trend has been the case with purpose-built bizjets. Even the ultralong-range jets of two of Falcon’s competitors,
Gulfstream and
Bombardier, manage with two engines, but Dassault has stuck with three, even on its latest design, the 7X.
Is it sense or stubbornness? Dassault, not surprisingly, says it’s the smart move. For the same reasons a three-engine airliner makes sense, a bizjet with three engines does as well. In addition to having added redundancy and safer engine-out handling manners, a three-engine airplane has better one-engine-out range characteristics, which is really important when that airplane is flying over the middle of an ocean — or somewhat before or after the middle — when it loses an engine. Having two-thirds of your engines left to “limp” home on is greatly preferable to having just half of your engines left to do that same job. Not to mention its noteworthy engine-out climb performance — again, the two-thirds rule — and its impressive hot-and-high and short-field takeoff performance. Finally, whereas an airliner can be counted on to fly to a handful of destinations throughout its air transport life, for bizjets, variety is the rule, and three engines, great short-field capability and exceptional hot-and-high performance all fit this profile perfectly."
http://www.flyingmag.com/aircraft/jets/falcon-900lx