What even happened in Season 3 that was of substance?

I'm about halfway through S4 now and I find it extremely difficult to even remember anything from Season 3 other than the vague description of 6 people who got stuck on a mountain in Chile and that it's probably the same Lovecraftian civilisation as the one on Svalbard. I don't mind a slow burn, but the burn was so slow and the payoff so unsatisfying that nothing really stuck.

Just so I have somewhere to come back to in case I get lost going forward, what are the most important plot points that happened in Season 3?