[finnn]

there are matrix bridges for all three of those

https://github.com/tulir/mautrix-whatsapp

https://github.com/Half-Shot/matrix-appservice-discord

https://github.com/matrix-org/matrix-appservice-slack

[patrakov]

The whatsapp bridge is based on the whatsapp-web api, which is non-functional if your phone is off. The phone part is what needs reverse engineering and replacing.

[rsa25519]

The Discord and Slack bridges use the respective platforms' official APIs, fwiw