Traction motors are mounted to one side of the driving axle. The centre axle is offset because there needs to be more room on one side where the centre motor is mounted. Like this:
Om mO mO
To make the axles equally spaced, the middle axle is either an unpowered idler axle to spread out weight, or the whole truck has to be much longer because there's a minimum amount of space required for the motors.
Chris van der Heide
My Algoma Central Railway Modeling Blog
It seems that first gen ALCO century sercies or old GEs have an off set three axle truck. Was the truck off set, with the two axles furtherest from center of the engine closer together? If so what advantage or disadvantage did this have? It would seem that it would have more wear on rails by having the wheels off set like this, or was it not enough to make a differance?