With AAD Connect, we no longer have a scheduled task that runs every 3 hour. AAD Connect has a built-in scheduler, which by default performs a delta sync every 30 minutes.
Although a synchronization now runs every 30 minutes, there may be occasions, where you still want to force a sync.
To do so, you launch Windows PowerShell (run as Administrator) on the respective server on which AAD Connect has been installed and type the following to import the AAD Connect PowerShell module:
To force a delta sync, use the following command: Start-ADSyncSyncCycle -PolicyType Delta
To force a initial sync, use the following command: Start-ADSyncSyncCycle -PolicyType Initial
To check scheduler settings , use the following command: Get-ADSyncScheduler
You can see sync issue on your Office 365 administrator page: