3000.chat is a chat platform utilizing the Matrix protocol.
Users can register a unique username and connect via cross-platform clients, using end-to-end encryption,
as well as using VoIP voice and video calls and the use of rooms for group messaging.
Matrix is an open protocol and allows the communication between different accounts by different vendors.
It is possible to register an account @userA:3000.chat which is able to communicate with an account like @userB:matrix.org or any other account which was created on a different matrix homeserver.
See the corresponding sections about how to register a username and how to use a chat client.