Act 3 boils down to a hunt for items. Specifically, you’ll need to find six Sunface Fragments. Each biome holds one fragment apiece, necessitating you travel back and forth between time periods. The fragments are permanent items that don’t reset upon death and the in-game menu keeps track of which ones you already have. Sounds like a simple task, right? Unfortunately, no.
Getting all six Sunface Fragments isn’t a simple task due to the roguelike nature of Returnal. The rooms that contain the fragments spawn randomly. You may get lucky and find the first three biome’s fragments in a single run and then have no luck in the final three. It’s a task that could take hours to complete if RNG doesn’t favor you. There’s also no way to know if your current run contains the fragments without searching every room in the biome.