Hi @miceagol and welcome to the forum 
You can click the pencil icon and change to “keep last”.
After the trigger node go to the if node (that filters for value - in this case open)
Then to the delay node, and finally the notification.
This means that whenever you receive a event on that trigger, it checks if the status is open, and continues to the wait node.
If for example the fridge were to be closed (during that 3 min periode), the already running flow will be stopped, and the condition will be evaluated again (from start). So in event of the fridge being closed, it will not “pass” the if condition, and therefore exit the flow.
If there is no new event (aka the door is left open), the timer will eventually run out which triggers the notification.