some of a system threads show start time about 3 hrs behind the current time.

what does this mean ?

If you mean some thread’s start time is 3 hours earlier than the current time, that means it’s been running for 3 hours.
Normally it’s a deadlocked thread, or hanging thread waiting on something that never happened

i dont see any error message about deadlock. These threads with 3 hours earlier time than current time, appear to have complete after 3 hours.

it is possible to figure out when these threads are delayed by 3 hours ?

If the thread is not running anymore, it’s almost impossible to know.
you can try to check Service log (if the long running service are configured to do service logging).

If it’s still running, you can check “Service Usage” to see if any other service is also running, from there you may figure out what’s causing the deadlock/hanging.