When will the streaming capability be available...

When will the streaming capability be available for the Series 3?

The music streaming capability via cellular data for Apple Watch Series 3 (GPS + Cellular) models will become available following a future watchOS release.

I'm afraid that no-one here can tell you anything about Apple's specific, unannounced plans in respect of timing. You are not talking to Apple here - this is a user-based technical support community.