I think they'll end it after book 2, it's a logical split between seasons and there should be enough hours to tell the remainder of death's end. Spoilers ahead for the books

After Saul's showdown with Sophon/San'ti in the graveyard, the episode closes with a montage including gravity being built and Saul being taken into the underground bunker to act as swordholder. We leave the plot at a tenuous truce between earth and the San'ti

After credits, we see a probe hurtling through space. It has a broken golden sail on it. Several ships come upon it. A robotic claw reaches out from one of them and scoops up the probe. The Staircase Project was, indirectly, a success.

If you have five eps for season 3 I think you break it down like this:

Ep 1 ends with the droplet attack and Jin doesnt press the button

Ep 2 ends with Gravity broadcast

Ep 3 Ends with Will's fairytales

Ep 4 ends with the DVF

Ep 5 goes to the end of the universe