I don't see why this isn't included, disappointing; the kit costs enough as it is.
While this feature is NA for Vivoactive 3, the workaround for now that is Settings>Sensors & Accessories>Heart Rate>Scroll to select Broadcast and tap onscreen message plus Green Arrow:
Now your Garmin Watch will broadcast HR data live to the Garmin Device (Edge 130 in my case) and the HR data will also show up on-screen during activity.