What model Garmin device are you using? What type of Activity is this? Are you using any sensors?
I want to see KM/H not MIN/KM and want to see how fast I run also in KM/H
Looks like it's not possible?
Depends where. On the watch self, you can usually customize the data screens in the way you want, but I am not familiar with Vivoactive 3. Check out the User Manual, if you do not know whether you can customize the Activity screens, and how.
As for the phone app GC Mobile, it shows both - the pace (min/km) as well as the speed (km/h) - look at the second tab of the activity.
And in Garmin Connect Web, you can toggle between the Pace and Speed modes at every activity.
You can also temporarily change the activity type from Run to Bike, if you wish the default mode in speed. Pace is the standard for running.