If we assume that this is a well designed pump from a reputable manufacturer, I would have a tendency to discount a pump design problem, particularly because all three pumps are failing at the same intervals and in the same location. It is highly unlikely that 3 or more pumps would have a weakness in the same spot.

Many years ago I stopped worrying about radial loads on a double suction pump. This is frequently a problem with an end suction design, but I have never seen such a problem on a D/S pump. This tends to point me towards a shock loading of some sort that would be a result of a system imbalance or improper or inappropriate operation. i.e. slamming a valve shut on the discharge line when operating at full speed, etc.