Why is my Garmin Forerunner 645 Music needing to pair at least 3 times every hour with my Android phone ?

May be because of a number of reasons. I recommend

• Remove and Add a Device From the Garmin Connect App | Garmin Customer Support Especially do not forget to remove the watch also from the list of Bluetooth devices in Android settings (important !!!) before reconnecting.
• Turn off any other BT devices in the proximity when pairing
• Do not connect the watch to more than one device at a time
• Doublecheck GC app permissions in Android, and diverse battery optimizing options - they often break the BT communication
• Especially some Samsung phones are know for such problems. Make sure you run the latest OS version for your phone. Sometimes resetting the Network helps. In other cases the WiFi on the phone needs to be turned off, or changed to 5GHz. There are some other tips (both for Samsung as well as for other Android brands) in the Sync / Connection Troubleshooting Guide.  

If nothing of it helps, reach out to the Product Support through their website at https://support.garmin.com. Use the Phone or Chat options for a fast reply. Email exchanges take a lot of time.