This chapter contains one of the best known numerical puzzles in the book, and the puzzle is entirely a matter of how two characters are read.
The phrase repeated three times can be taken as three out of ten, giving a distribution of the population into thirds: three in ten are companions of life, three in ten companions of death, and three in ten are people who, living, move toward the ground of death because they live life too thickly. On that reading a tenth is left over and unaccounted for, which some readers take to be the sage of the second half.
The same phrase can also be taken simply as thirteen, and an old commentarial line does exactly that, glossing the thirteen as parts of the body, the four limbs and the nine openings, so that the chapter is about the routes by which life and death enter a person rather than about proportions of a population.
Both readings are ancient, both are grammatically available, and the choice determines what kind of chapter this is: a piece of social observation or a piece of physiology. English versions have to pick, and the pick is usually invisible to a reader who has only one version.
The third group is the interesting one on either reading. These are not people who die of misfortune or violence. They are people who move toward death by the intensity of their attachment to living, which is the fault chapter fifty-five calls adding to life and calls ill omened, and which chapter seventy-five attributes to those above.