That 3 minute delay is probably normal depending on how you are testing.
It won’t happen ever time, but it will happen if you repeatedly open the sensor. A rundown of how sensors report activity monitoring status in Alarm.com is linked below.
Door/window sensors will cause an immediate notification if activated. But then there is a 3 minute delay between then and when the same status change will be registered.
So for example, the first time you open the door you should get a quick notification. But then if you immediately close and open it again 20 times, you will only see one more notification 3 minutes later.