Can insight be gained at any point in insight meditation, or do you have to go through all 7 purifications of insight? Can insight into impermanence suffering and not-self be gained at any point in insight practice or do you have to go through all 7 purifications of insight?
The seven purifications are not all related to insight; only the sixth specifically deals with the realization of three characteristics. The first purification is related to morality, and the second to concentration. The seventh is super-mundane, so is not related to realization of the three characteristics. The fourth and fifth do somehow relate to the three characteristics, but the fourth has more to do with understanding cause and effect, and the fifth to do with finding the path rather than walking it.