From the manual construct.net/en/make-games/manuals/construct-3/plugin-reference/multiplayer
LEARNING TO MAKE MULTIPLAYER GAMES
Designing Multiplayer games is challenging. It is essential to follow the introductory tutorial series to gain an understanding of how to use the Multiplayer object correctly. You are likely to struggle if you try to skip ahead and start making a game without fully understanding how various aspects of multiplayer games work. Mistakes can also result in degraded gameplay quality, with unnecessary lag.
The four introductory tutorials are:
Multiplayer tutorial 1: concepts
Multiplayer tutorial 2: chat room
Multiplayer tutorial 3: pong
Multiplayer tutorial 4: real-time game
(Note these tutorials refer to Construct 2, but the principles are the same.)
You can also find examples of the Multiplayer features by searching for Multiplayer in the Start Page.