You better ask this on the Vivoactive 3 forum, since here not everyone is famliar with your watch. It also depends whether you make the workout with an activity started on the watch, or without any. Generally, on most Garmin watches, you can customize the look of the main watch-face, if you do not use any activity. And if you use an activity, then adding a HR screen to the activity, and scrolling to it, should work. At least I believe that my watch stays on the selected screen during an activity as long as I let it.
