Of the contiguous 48 United States, only 13 states allow motor carriers to pull triple trailers. The Dakotas (North and South) are two of the states and my employer uses specially trained drivers to pull three 28 foot trailers. These Long Combination Vehicles are thoroughly inspected before, during and after driving to it's destination. The flexibility of these triple trailer setups can be very advantageous. Each trailer can be loaded to different destinations and dropped off without having to open the trailers and have freight removed from them in order for the driver to continue to the next destination.
Here is an example of a triple trailer setup:
The significance of the cache is precisely because of it's location, which is approximately 2 miles into South Dakota. The nearby states of Iowa and Nebraska do not allow triples, so many companies must find a place to reassemble their equipment in order to follow state regulations in the state they are serving.
Three drivers from Kansas City for example can only bring 2 trailers each to this location since they travel through Iowa, while 2 drivers coming from Fargo can bring 3 trailers each and is the meaning behind the title. The 3 Kansas City drivers rearrange their 6 trailers into triples to continue going north, while 2 Fargo drivers rearrange their 6 into three sets of 2 to go back to the south. This rearranging is usually done within view of this cache several times a day, which has plenty of room to park and grab from a car or a large commercial vehicle. Also of note is the food, coffee and even casino options nearby. Enjoy!