Six-wheeled automobiles are not only Mercedes G63 AMG 6X6 or exclusive Dartz. There even existed a three-axle Soviet pickup GAZ-M21 from 1937 (not to be confused with the Volga). 
Experiments with small six-wheeled cars at GAZ began in the early 30s. Initially, they used units from the passenger GAZ-A and the truck GAZ-AAA, and later the more modern GAZ-M1 became the donor. 
The M model contributed a 50-horsepower engine, front axle, brakes, and part of the cabin.
The maximum speed was 87 km/h, and the carrying capacity was 950 kg. 
Interestingly, there were small rubber wheels under the thresholds to help in off-road conditions. Incidentally, only the two rear axles were driven. 
The six-wheeled GAZ-M21 demonstrated phenomenal cross-country capability, as seen in archival photos. The pickup was recommended for production both for the national economy and the military and even a trial batch of 100 vehicles was assembled.
However, after 5000 km of testing, it turned out that the frame and suspension could not withstand extreme loads, and the non-driven front wheels often bogged down. In general, it was decided to cease production and focus on developing the all-wheel-drive GAZ-61-73. 
By the way, there also existed a six-wheeled sedan GAZ-25. Not so long ago, one such car was sold at the price of a supercar. TopGir via Carakoom