I don't think double versus triple is worth arguing about, since in Rv's, everyone will know what you are talking about.
But just for reference, in CA, you need a "Doubles Endorsement" on your class A commercial license, in order to pull two trailers. There is no triples endorsement, since CA does not allow pulling 3 trailers.
Also, if pulling two trailers is triple towing, then I guess all of us who just pull one trailer must be doubles towing, right? Just doesn't make sense or sound right.
But the fact is, plenty of people will call it triples, and I always know what they mean.