Actually, a 3 wheeled design is inherantly UNstable: there is a reason why all the trike ATVs left the market in a flood of lawsuits.
No, they left the market because of staged "yellow journalism" stunts on 20/20 and 60 minutes, et. al.
Much like the BS over the "exploding" Chevy/GMC pickup fuel-tanks.
I and several of my friends rode them constantly through the '80s - never once managed to turn one over, no matter WHAT stupidity we did with 'em. (but did come close a few times!) The major "problem" I saw with stability on these things was an absolute lack of any real suspension for the back - a couple of trailing-arms (sometimes!) & live axle and the "sponginess" of the underinflated rear tires was the extent of anything I remember on any of of those things.
By the time they developed any kind of independent rear-suspension, it was a moot point - they were "gone", and all that effort went to the 4-wheelers.
And of course, manufacturing only quads has made the offroad-powersports industry easily 110% safer. (/sarcasm)