you couple the third trailer to the second trailer before you couple them to the first trailer.
A triple combination vehicle has four units (not counting the converter dollies): a tractor and three trailers. You couple a triple as follows: 1. Couple the first trailer to the tractor the same way that you couple any combination vehicle. 2. Couple the third trailer to the second trailer using the method for coupling doubles (which will involve the use of a converter dolly). 3. Move another converter dolly into position. 3. Couple the tractor and first trailer to the second and third trailer using the method for coupling doubles (which will involve another converter dolly).
B you couple the second trailer to the first trailer and then couple them both to the tractor.
C you do not have to use a converter dolly.