Be careful with "Trigger once", it's the most misused feature in Construct and it can cause lots of problems when used incorrectly. Don't use this condition by its own, with loops, inside of functions, triggers, with conditions that can pick multiple instances etc.
what's the difference between using "PickedCount = 0" and using "Else"
When nothing is picked in event #3 in my screenshot, you will not be able to check "Sprite.PickedCount=0". If you add any sub-events or actions inside this event, they will not be executed.