SDCL § 32-22-10: Requirements for combination vehicles.

Combinations of vehicles of not more than three units may be operated upon a public highway if the combinations consist of truck tractor--semitrailer--semitrailer converted to full trailer by use of a dolly equipped with fifth wheel which is considered a part of the trailer for all purposes and not as a separate unit; truck tractor--semitrailer--full trailer; or truck tractor--semitrailer--semitrailer. If a combination consists of more than two units, the rear unit of the combination shall have breakaway brakes.