3,678 lbs is the curb weight (without passengers and cargo)
Single vehicles with a Gross Vehicle Weight Rating of more than 26,000 lbs., and combination vehicle with a Gross Combination Weight Rating of more than 26,000 lbs., when the vehicle in tow does not have a weight rating in excess of 10,000 lbs.
If it's a vehicle with a Gross Vehicle Weight Rating of over 26,000 lbs., then yes, you do.
Only if the vehicle has a Gross Vehicle Weight Rating of 26,000 lbs. or less (and remains under that weight). Otherwise, you need a CDL.
If the vehicle has a Gross Vehicle Weight Rating of more than 26,000 lbs., yes. If not, no.
If it has a Gross Vehicle Weight Rating in excess of 26,000 lbs., yes.
If the vehicle's Gross Vehicle Weight Rating is in excess of 26,000 lbs., yes.
If the Gross Combination Weight Rating (the combined Gross Vehicle Weight Rating of the truck and of the trailer) is in excess of 26,000 lbs., and the vehicle is of a type which does not fall under an exemption to CDL requirements (emergency vehicle, registered farm vehicle, military vehicle, or recreational vehicle), then yes, you need a CDL.
If it's under a Gross Vehicle Weight Rating of 26,000 lbs., whatever the standard driver's license class is for your state, although you will need a DOT medical card. If it's a single vehicle with a Gross Vehicle Weight Rating in excess of 26,000 lbs., you'll need a Class B CDL. If it's a combination of vehicles with a Gross Combination Weight Rating in excess of 26,000 lbs., and the vehicle in tow has a Gross Vehicle Weight Rating in excess of 10,000 lbs., you'll need a Class A CDL.
If the vehicle has a Gross Vehicle Weight Rating of more than 26,000 lbs., yes - and you would in every state.
If it has a Gross Vehicle Weight Rating of more than 26,000 lbs, then yes.
That depends on the Gross Vehicle Weight Rating of the truck, not on the length.
Length is irrelevant - Gross Vehicle Weight Rating and usage is what determines this.