Is there some reason that a car/truck designed to be "flex-fuel" won't run on "propane/CNG/LPG" as well as a "standard gasoline engine"?
You would need a separate fuel system from tanks to injectors much as if you did a gaseous conversion. It is costly. If you had both storage/fuel systems you could run on all three, but the price would be higher.
Go here: http://www.flexdi.com
On the air assisted side of their injector, they could run the gaseous fuel through it for only one injector per engine. These guys have got one trick solution.