As we have already posted elsewhere and often, the ONLY "definition" that matters is the "DOT" or "Federal Motor Carrier Safety Administration (FMCSA)" definitions.
The "laws" do NOT "vary from State to State", but are UNIFORM across the Nation. This has been so since the 80's.
Here is a visual to better explain what is meant by Single, Double, & Triple .......
#1 is a SINGLE.
#2 thru #6 are DOUBLES.
#7 is TRIPLES.
It is HOW MANY TRAILERS you are PULLING, not how many "units" in the vehicle.