Triple typing would be a balance nightmare. I seriously doubt they'll do that.
While I do agree, the recent leak mentioned dualtype moves. Clumsy implementations of that would be an even bigger balance nightmare... so hopefully they did it right!!
According to some other guy, the supposed dual-type moves are moves that are classified as one type but deal damage as if it were a different type. He mentioned a Ground type move which deals Ice type damage.
Or, that part of the leak is wrong. Whatever.
So it receives STAB from a Ground Pokemon but is super effective against a Dragon, Plant, and Flying?
That's a pretty neat way of doing dual-type moves. Gives certain type a way of striking resistant opponents without sacrificing STAB.
To me, that's the only sensible way to do a 'dual-type' move. Not so much different from Psyshock and Psystrike (which are Special but deal Physical damage).