Triple threat? What games exactly. I only know of a few that would qualify.
Well, a "triple threat" would imply exactly three. AM2R, Pokemon Uranium, and No Mario's Sky, I'm guessing. Super Mario Fusion shut down (I think they were just looking for an excuse more than anything), and Mother 4 announced that they would change the name, but neither received direct legal threats, or so I understand.
There's so many fan-made clones of Super Mario on websites, they couldn't really attempt to sue all of them.
I think they did go after Super Mario Bros. X. Seems to me they need to strike a careful balance between being protective and calling attention to things that they hope will just get swept under the rug.