Chapter III of Season 12 went by really quickly. The reason why is simple. I was able to complete the majority of the Chapter III objectives long before I was able to unlock Chapter III.
Players can complete five objectives by finishing Bounties in each Act. You have to complete one set of Bounties in order to complete an objective in Chapter I. You also need to collect Legendary crafting materials in order to put things into the Horadric Cube – and the only way to get them is to complete a set of Bounties for each Act.