The game already struggles with desynchronization, invisible objects, and bugs even with just two players. Adding a third would likely break world-state syncing.
Community discussions on Steam and the official forums highlight several reasons why a 3-player mod hasn't been realized:
While you can’t add a third person, you can use mods to significantly overhaul the 2-player survival experience. stranded deep 3 player mod
Supports 2-player online co-op and local split-screen.
Currently, there is for Stranded Deep. Unlike games with highly flexible engines like Minecraft or Valheim , Stranded Deep’s multiplayer architecture is deeply tied to a peer-to-peer (P2P) system designed specifically for two people. Technical Hurdles Supports 2-player online co-op and local split-screen
Many of the game’s core systems—including the starting life raft and resource spawning—are hardcoded for a maximum of two occupants.
For a complete breakdown of how the existing co-op systems function and the challenges you might face, watch this guide: Stranded Deep Co-op | What you should know in 2023 YouTube• Jan 25, 2023 Can You Use a Mod for 3 Players? The game already struggles with desynchronization
Stranded Deep ’s isolated, sun-drenched islands and shark-infested waters provide a visceral survival experience. While the game was originally a solo journey, developers eventually added to allow players to face the Pacific with a friend. However, many players find themselves with a group of three or four, leading to the persistent question: is there a 3-player mod for Stranded Deep? The Current State of Multiplayer