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.
Some insight into 3 characteristics is gained at every stage, though the depth (and thus the transformative power) of this insight is different, and also type of objects in which 3c are seen are different as one moves through the stages of insight. One goes forward through the stages by understanding more and more of 3c.