what defines a 3/4 ton?

Put any tag you want on the truck, here is what the government looks at US truck class:
Light-Duty Trucks

The light-duty trucks category includes commercial truck classes 1, 2, and 3.

Class 1: This class of truck has a GVWR of 0–6,000 pounds
Class 2: This class of truck has a GVWR of 6,001–10,000 pounds
Class 3: This class of truck has a GVWR of 10,001–14,000 pounds
Medium-Duty Trucks

The medium-duty trucks category includes commercial truck classes 4, 5, and 6.

Class 4: This class of truck has a GVWR of 14,001–16,000 pounds
Class 5: This class of truck has a GVWR of 16,001–19,500 pounds
Class 6: This class of truck has a GVWR of 19,501–26,000 pounds

Heavy-Duty Trucks

The heavy-duty trucks category includes commercial truck classes 7 and 8. Drivers of vehicles in these classes are required to have a commercial driving license (CDL) to operate the vehicle.

Class 7: This class of truck has a GVWR of 26,001 to 33,000
Class B required
Class 8: This class of truck has a GVWR of greater than 33,001 and includes all tractor-trailers Class A required