I discovered long ago that I had a diesel loco (Athearn Genesis I believe it was) that had 3 axle trucks with very small flanges. It would be one of my first locos to derail if I had trouble spots. I inspected the situation very closely and discovered that with the slightest variation in rail heights within short distances (or over turnout points), the truck would choose to balance on only two of the axles, the two closest ones such as the center axle and one of the ends. That left the other end of the truck just slightly 'up in the air' where its small flange wheels could NOT keep it from riding over the railhead.
That loco became my track laying tester. It could find my errors more precisely than laying a straight edge across the tracks.
3 axle trucks can be troublesome, and possible worst if the truck frame is too flexible,...adding another dimension into the equation !