Kids, don't do trimerism. It's bad for you.
Xorn come in three sizes, continuing the pattern seen in arrowhawks, salamanders, and tojanidas. Regardless of age, they have high natural armor (we're talking +12-16 here), immunity to cold and fire, resistance to electricity, tremorsense, DR 5/bludgeoning, a burrow speed with Earth Glide, and all-around vision, which I must admit is a pretty neat set of defensive benefits.
For 3 RHD, the aforementioned benefits are quite nice, and minor xorns have a strength bonus (despite being Small), a very strong bite attack, and three supplementary claws. +12 natural armor is impressive as well.
Even if their special abilities aren't anything to write home about, I feel like minor xorns will have a considerable impact on combat, and in addition their RHD are low enough to allow access to prestige classes without the usual struggles. For now, I'll give it +1 LA.
Two points of strength, a size increase, +2 natural armor and bigger natural weapons, plus Cleave as a bonus feat for some reason. Not worth four extra RHD, that's for sure. -0 LA.
Actual stat increases this time, but even bigger RHD increases. Also, 'walking through rock' isn't sufficient noncombat utility at these levels, if it ever was at all. -0 LA.