Season 1 was slow paced, which can be excused to some degree as characters get introduced each episode with ominous world building.
Season 2 followed the same path as season 1, with slow individual stories, though it had at most 4 moments when the overall story connecting the characters started to pick up pace and ended with an interesting cliffhanger.
Season 3 was a literal drop from that cliff. Too much attention on individual stories, which failed to coalesce or match with the build up of the previous season. If the ending of this season was aimed to be a cliffhanger then it failed.
Final thoughts :
Heavy on exposition, wispy on narrative. The overall scarcity of aliens tests the patience and intelligence of the viewer.