My steps are not syncing on my vivoactive 3 since 25th September, even the challenges steps have been static.

There are basically three possibilities: 

1. The BT synchronisation on the phone app does not work (the white circle around the watch symbol never closes) - in that case follow the tips in the Sync / Connection Troubleshooting Guide. Alternatively, you can sync the data with the GC account over the USB cable and Garmin Express on a compuiter, or manually by importing files from the device folder //GARMIN/ACTIVTY/ to the Import Data page of GC Web (in case of errors during the import, see the hint #3)
   
   

2. When the synchronisation works (the white sync circle closes and turns green), but not all the data is appearing in your account, then it very probably means that your device is associated with multiple accounts, and a part of the data lands in the other account. It frequently happens for example when you reinstall the application, and inadvertently sign up a new account instead of signing into the original one. Follow the instructions from the support document Data is Missing from My Garmin Account After Syncing to resolve the issue.
   
   

3. The storage (or a FIT file storing the Activity details or HR data and the steps data) is dammaged or full. Try backing up and deleting old files from the folder //GARMIN/Activity and //GARMIN/Monitor. Look up the last unsynced activities from the folder //GARMIN/Activity, and try importing them manually on the Import Data page of GC Web. If you get errors when trying to import them, try fixing the broken files by passing them through the Section Remover of FIT File Tools (without removing anything), and then importing again the repaired file(s).