That requires access to the actual phone and the unlock code in the case of WhatsApp (you need to identify to add a WhatsApp web client).

For telegram it's a bit easier yes, but the user can set up an additional password. I have done so of course. Note that telegram is not E2EE so they can give your stuff to the police at any time unlike WhatsApp and signal.

For signal I don't know as I don't really use it but i understand it works the same way as WhatsApp, scan a QR code and authenticate to the phone.

Also, with all 3 systems it's clearly visible when you look at the linked systems.