Damn! Dark, the trilogy as a whole is a very well written show with a captivating story. This only becomes more apparent when you realize how consistent the series has been throughout the three seasons and how everything was so well thought out beforehand. Granted, calling it immensely complicated would be an understatement as it may require careful attention or multiple viewings to grasp and remember everything but once you clear this hurdle and understand this show, its brilliance will readily become apparent.
This show may very well go down in history as one of the greatest Sci-Fi content both in terms of films/tv ever made. We will be darn lucky if anything ever comes close to the greatness of this show in future. Or maybe considering how it is a sleeper hit, it inspires studios to order more thought provoking contents, but who knows.


[spoiler] I think , once they prevent the deaths and time-travel from being invented, Jonas'(Split Reality 1) and Martha's(Split Reality 2) universes(realities) are annihilated, the Scientist's "Original reality" splits/branches again, to form another "3rd" Split Reality which is the one shown in the end.
Since "Endings are beginnings and beginnings are endings or everything is predetermined and past/future can't be changed", this would make the most sense without breaking the show's rule because if the Scientist didn't try to invent time Travel, there wouldn't be 2 split realities and with that no Jonas or Martha coming to change the past, and if they didn't come, he would invent time Travel, a Paradox.
So the original reality still exists alongside the new 3rd reality. However Jonas and Martha are erased from existence since their universes no longer exist.
Hannah deciding to name the unborn child 'Jonas' was also a nice way to spare us a completely sad ending.
But hey who knows! The series is open for multiple interpretations.[/spoiler]


Here's something you might find helpful:
_**How To Understand Dark**_
1. Determinism:
**I.** Watch Predestination (https://www.imdb.com/title/tt2397535 ) to acquaint yourself with the general idea of a closed time loop. Also maybe give Triangle (https://www.imdb.com/title/tt1187064 ) a shot.
**II.** Arrival is also great.
**III.** Watch Devs (https://www.imdb.com/title/tt8134186 ) [Ignore the last episode, unfortunately Alex Garland still can't stick a landing] to really get the essence of a deterministic universe on the quantum level.
2. Split/Parallel Realities:
**I.** Rick&Morty has several episodes dealing with the concept. However, S02E01 A Rickle In Time(https://www.imdb.com/title/tt4462494 ) and the recent S04E08 The Vat Of Acid ( https://www.imdb.com/title/tt10655692 ) both do the best job of explaining IMO.
**II.** Steins Gate is a masterpiece if you are into anime (https://www.imdb.com/title/tt1910272 )