How can I find the add friends tab in Apple Watch series 3?

Hi

Under watchOS 4.x, the Friends feature (which was available under some prior versions) is no longer included and there is neither a direct replacement for it nor a separate Contacts app on your Apple Watch.

When placing calls, sending new messages or sending emails, contacts can instead be selected:

From within the Phone, Messages and Mail apps:

• Calls: Make phone calls
• Messages: Send messages

Or, for calls and messages, by using Siri:

• Use Siri on your Apple Watch - Apple Support

You're welcome! 🙂