never heard of triangular rods and I've always sharpened my serrated knives with a steel, just like any other knife. It works perfectly and I've never had the slightest problem slicing bread etc etc...... If you wonder "why triangular rods, not round ones like some others ?" It's because there are a lot of serrated blade knives out there and those need to be maintained too.
.......
Yup.Butchers steel all the time but they still have their knives sharpened regularly.