Notwithstanding the provisions of RCW 46.44.036
and subject to such rules and regulations governing their operation as may be adopted by the state department of transportation, operation of the following combinations is lawful:
(1) A combination consisting of a truck tractor, a semitrailer, and another semitrailer or a full trailer. In this combination a converter gear used to convert a semitrailer into a full trailer shall be considered to be a part of the full trailer and not a separate vehicle. A converter gear being pulled without load and not used to convert a semitrailer into a full trailer may be substituted in lieu of a full trailer or a semitrailer in any lawful combination;
(2) A combination consisting of a truck tractor carrying a freight compartment no longer than eight feet, a semitrailer, and another semitrailer or full trailer that meets the legal length requirement for a truck and trailer combination set forth in RCW 46.44.030
(3) A motor home or travel trailer with a cargo extension, provided that there are no trailers or secondary cargo extensions or units attached to the cargo extension.