What is the command to sign a message using a segwit private key using electrum 3? I want to sign and verify a message using a segwit private key that i generated with another application. How can I sign and verify a message using electrum 3 cli?
Electrum provides signmessage and verifymessage commands that you can use to sign and verify messages.

For signing messages with a different private key, you will need to import it into the wallet.

Verifying a message does not require any additional work.