I found when i was rearing mine that when one of the three start getting bigger than it's siblings, it is time to move it to it's own tub to prevent it from eating it's siblings.
Once the little piggy has been removed it will give the smaller of the group chance to get food. In this situation i would also put in a pre killed cricket with the smaller siblings to allow them to have an easy meal
It is a case of survival of the fittest basically. It sounds like it was more than likely starvation that caused the death.
I also found with mine that after the first shed their growth rates varied quite a lot, some ate all they could and others were a bit slow to get food.
It does mean keeping an extra eye on them but it's worth it to get a good percentage to 3i. By then they should make it to adulthood fine.
The birth to 3i bit seems to be really hard to get them to survive, but a bit of tlc should do the trick