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.
Which is also true if you convert any existing "non-flex-fuel" vehicle to use propane/LPG/LNG. So, I have to ask again---why "not" require that any vehicle produced in or imported to the US be "flex-fuel" qualified. IIRC the Chevy S10 pickup is already a flex-fuel, and I'm pretty sure there are others. How many, I dunno, as I've not studied the issue.
The technology at your link looks interesting.