Serious Sam: The Third Encounter picks up where The Second Encounter ended
Croteam and Devolver Digital announced it for PC on Steam. It's in early development, with no release date.

Croteam and publisher Devolver Digital have announced Serious Sam: The Third Encounter, the next game in the arcade shooter series. Gematsu reported the announcement on 21 October.
The Steam page calls it a direct continuation of Serious Sam: The Second Encounter, the 2002 game: "Not a prequel. Not a side story. A true continuation." Sam takes the fight to Mental's horde across new alien worlds, with classic weapons and new ones.
You can play solo or in co-op. Croteam says it's being made by the team behind The First Encounter and Serious Sam: Siberian Mayhem.
It's coming to PC through Steam and is in early development. There's no release date, price or console version yet.