Earlier quoted context omitted.
Signal does have sync, but it works exactly the same way as Whatsapp's does, requiring your phone to be on and connected Telegram's secret chats are also client-specific and not synced, and I have no idea how it would even be possible to have e2e chats synced on multiple devices without having a "main" device that's doing the actual crypto or sharing the secret key
If you're interested in learning how to implement end-to-end chats with multiple devices, checkout out Wire security whitepaper ( https://wire.com/resource/Wire%20Security%20Whitepaper/downl... ) for one way of doing it. Hopefully, OpenWhisperSystems will release documentation about their implementation in Signal (note: it doesn't require phone, unlike WhatsApp)
I got fooled by Signal's requirement of a phone for the activation and thought it was also required for normal usage, I guess that's better than Whatsapp, though being able to register and use it without a phone at all would be even better (even Telegram doesn't do this though, IIRC it requires a phone number)