On my triple axle, have never had a problem with any position tire after going to Michelin LT Ribs.
Before going to LT's, didn't matter what position the tire was on the trailer. All position ST tires would fail on my triple axle, regardless of tire position.
I don't see any difference in my LT Ribs wear comparing my middle tire to the front or rear tire on my TH. They all seem to wear the same (very little), and tow smooth and steady all the time...
Think the key here is, if you use the best grade tires, should not have any problems with a big triple.
With my bumper pull triple I don't think I can make as tight a turn as a 5th wheel. In fact I can't(or don't) turn sharp enough to cause my middle tire to be the pivot point. The trailer will always be moving a little forward or backward while turning. Thus preventing the middle tire from being in an extreme pivot point non rolling scrub only position.
I am far from a expert on this, but this has been my experience.
jmo