This review is based on watching the season through episode 18 (of 26) "The Last Heartbreak" inclusive.
"Lucifer" wasn't precisely a "good" show in its first two seasons, but it was entertaining and quite funny. But abruptly in its third season it all goes sideways. Lucifer (the character) has forgotten every lesson he learned in the past two years, and has reverted to being nothing but the egotistical asshole who doesn't know how to deal with humans that we met in the first episode of the series. He's also become significantly more stupid - more impulsive, more petty, and less aware of things that happen around him. Amenadiel and Maze suffer much the same fate. Most of the other characters are just plot drivers: their behaviour isn't consistent from episode to episode, but changes to suit the needs of the writers. The new plot driver for the season is Cain - the first murderer, cursed with immortality. The guy who plays him (Tom Welling) is good looking, a bad actor, and written utterly unconvincing as someone thousands of years old: unlike Lucifer, Amenadiel and "Mom" (from season 2), he's been leading a human life ... and apparently the only side effect of multiple millennia of life is to be suicidal (except he can't die). I'm not buying their interpretation of the character.
Side note: I'm especially not buying this after seeing "He Never Died," which was an immensely better cinematic take on the whole idea Cain punished with immortality ... A movie that premiered March 2015, before Lucifer's first season, never mind Lucifer's take on Cain.
Returning to "Lucifer:" Lucifer (the character) is taught lessons this year (very bluntly I might add, subtlety was never a strength of the show and now it's applied even less) - and yet he doesn't remember them from episode to episode. I also didn't believe Maze being so viciously jealous over a relationship she chose to quit: like Lucifer, she was learning in the previous two years, but the writers chose to revert her behaviour because it cranks up the "drama" (really, it doesn't ...) and the "comedy" (again, no). The quality of the writing is much poorer: I don't know if this was budget cuts or the increase in the number of episodes (from 13 to 18 to 26 this year) or a combination of the two, but it's just ... bad. And sadly, and rather importantly, it's significantly less funny.
I suppose that it could be argued that this was a soap opera from day one, but in the first two seasons it managed to retain some of the better qualities of that genre. No more.